Some party insiders believe that Mr Johnson’s presence in Parliament will destabilise Mr Cameron because there will be so much focus on whether he intends to challenge for the Conservative leadership.

However, friends of Mr Johnson on Wednesday said that “it is not Boris’s aim to topple Cameron”.

“He who wields the knife never gets the crown – he knows that,” the source said.

“Boris believes that Cameron will pull it off [at the election]. He needs to be seen as a significant player delivering that success.”

Mr Johnson’s return to Parliament is sure to lead to tensions with George Osborne, the Chancellor, who is said to harbour his own leadership ambitions.

Michael Gove, the former Education Secretary and a key ally of Mr Osborne, earlier this year said Mr Johnson would not make a suitable prime minister.

Asked about the threat of Mr Osborne and Mr Gove, one ally of Mr Johnson said: “Boris outclasses them all. People will be able to see Boris’s record.”

The Prime Minister has previously signalled that he could be prepared to give Mr Johnson a Cabinet post after he finishes his mayoral term, saying that he would be “potentially very good for the Government”. Mr Johnson said on Wednesday it was “largely” Mr Cameron’s idea for him to run as an MP.

“It was largely, obviously, at his instigation that I decided to have a crack,” Mr Johnson said.

It is widely expected that the Conservatives will now attempt to use Mr Johnson as a “secret weapon” during the election campaign.

He will be deployed in seats across London, where he still enjoys strong support after six years as mayor.

One of Mr Johnson’s key strengths is his ability to connect to young voters, one of the Conservatives’ key aims leading up to the vote in 2015.

One Government source said: “Boris and the Prime Minister have done events together recently during the Newark by-election and before the local elections. They worked well as a team.

“Having him around will make a huge difference.”

Christopher Hope, The Telegraph’s senior political correspondent, questions Boris Johnson about the ministerial job he wants – and how much is a loaf of bread

He also said he will surrender his £67,060 salary as an MP, though he could also be legally forced to give up a large portion of his £143,911 mayoral salary if he joins the Commons.

The decision was orchestrated by Mr Cameron following years of rivalry between the two men.

However, Mr Johnson’s return to Parliament will pose a major challenge to George Osborne, the Chancellor, who is widely tipped to be planning a leadership bid when Mr Cameron stands aside.

Mr Johnson earlier this year clashed with Michael Gove, a key ally of Mr Osborne, after the former education secretary said that the Mayor would be “unfit to lead the nation”.

It could lead to a split in the Tory Party after the next election, with factions of MPs forming behind either Mr Osborne or Mr Johnson.

Theresa May, the Home Secretary, is also thought to be plotting a future run at the leadership.

Some insiders believe Mr Johnson’s return will destabilise Mr Cameron because it will lead to constant debate about whether the Mayor is attempting to unseat the Prime Minister.

“We have danced around it for an awfully long time now and, as you know, the Prime Minister has been clear that I can’t endlessly go on dodging these questions as I have tried to do so,” Mr Johnson said. “I have not got any particular seat lined up, but I think that in all probability I will try to find somewhere to stand in 2015.”

A Cabinet role, particularly in the business department, is seen by Mr Johnson’s allies as a key staging post in his bid to become Conservative leader once Mr Cameron steps down.

“Look at his record in London on transport and also on championing the city abroad,” one friend said. “Being a cheerleader for Britain as part of the Government and encouraging people to invest here is exactly the kind of thing he could do.”

Mr Johnson told the Telegraph: “I’ve got to deliver for London. I’ve got to fulfil the mandate and that’s got to take priority. Being in Westminster will actually be an advantage from the point of view of my job in London. I’ll be able to be an advocate for the city.”

Asked whether he would want a Cabinet post as Business Secretary, or a newly created role as infrastructure minister, Mr Johnson said: “Before any such discussion could take place there are still all these hoops I’ve got to go through. We’re still quite a long way off.”

Some Westminster insiders believe that if Mr Johnson is seen as an integral part of a Conservative victory next year, there could be an organised takeover in 2018 if Mr Cameron decides to step down.

However, one Cabinet minister said: “There will be no organised takeover in 2018 in Boris’s favour. He doesn’t have the support in the party and there’s a big object standing in his way in the shape of George Osborne.”

Mr Johnson repeatedly denied that he has any leadership ambitions. He said: “When David Cameron finally steps down, in 2030, or whenever, it may be that there’s a vacancy, but it will probably be filled by a person who’s a teenager now.” Mr Johnson’s announcement came after he delivered a fiercely Eurosceptic speech, saying Britain has “nothing to fear” if it leaves the EU.

Christopher Hope, The Telegraph’s senior political correspondent, questions Boris Johnson about the ministerial job he wants – and how much is a loaf of bread

He also said he will surrender his £67,060 salary as an MP, though he could also be legally forced to give up a large portion of his £143,911 mayoral salary if he joins the Commons.

The decision was orchestrated by Mr Cameron following years of rivalry between the two men.

However, Mr Johnson’s return to Parliament will pose a major challenge to George Osborne, the Chancellor, who is widely tipped to be planning a leadership bid when Mr Cameron stands aside.

Mr Johnson earlier this year clashed with Michael Gove, a key ally of Mr Osborne, after the former education secretary said that the Mayor would be “unfit to lead the nation”.

It could lead to a split in the Tory Party after the next election, with factions of MPs forming behind either Mr Osborne or Mr Johnson.

Theresa May, the Home Secretary, is also thought to be plotting a future run at the leadership.

Some insiders believe Mr Johnson’s return will destabilise Mr Cameron because it will lead to constant debate about whether the Mayor is attempting to unseat the Prime Minister.

“We have danced around it for an awfully long time now and, as you know, the Prime Minister has been clear that I can’t endlessly go on dodging these questions as I have tried to do so,” Mr Johnson said. “I have not got any particular seat lined up, but I think that in all probability I will try to find somewhere to stand in 2015.”

A Cabinet role, particularly in the business department, is seen by Mr Johnson’s allies as a key staging post in his bid to become Conservative leader once Mr Cameron steps down.

“Look at his record in London on transport and also on championing the city abroad,” one friend said. “Being a cheerleader for Britain as part of the Government and encouraging people to invest here is exactly the kind of thing he could do.”

Mr Johnson told the Telegraph: “I’ve got to deliver for London. I’ve got to fulfil the mandate and that’s got to take priority. Being in Westminster will actually be an advantage from the point of view of my job in London. I’ll be able to be an advocate for the city.”

Asked whether he would want a Cabinet post as Business Secretary, or a newly created role as infrastructure minister, Mr Johnson said: “Before any such discussion could take place there are still all these hoops I’ve got to go through. We’re still quite a long way off.”

Some Westminster insiders believe that if Mr Johnson is seen as an integral part of a Conservative victory next year, there could be an organised takeover in 2018 if Mr Cameron decides to step down.

However, one Cabinet minister said: “There will be no organised takeover in 2018 in Boris’s favour. He doesn’t have the support in the party and there’s a big object standing in his way in the shape of George Osborne.”

Mr Johnson repeatedly denied that he has any leadership ambitions. He said: “When David Cameron finally steps down, in 2030, or whenever, it may be that there’s a vacancy, but it will probably be filled by a person who’s a teenager now.” Mr Johnson’s announcement came after he delivered a fiercely Eurosceptic speech, saying Britain has “nothing to fear” if it leaves the EU.

Christopher Hope, The Telegraph’s senior political correspondent, questions Boris Johnson about the ministerial job he wants – and how much is a loaf of bread

Mr Johnson’s speeches at the annual party conference regularly empty the bars as delegates flock to listen and local parties will be desperate to be associated with him.

A key factor will be ensuring that the seat is within easy reach of London because he will combine being an MP and London Mayor for a year.

Mr Johnson repeatedly said he did not want the issue to hang over the party’s conference in October which suggests he wants the selection agreed by then.

Informed sources say that Mr Johnson is eyeing up the safe Conservative seats of Uxbridge (2010 Tory majority 11,216) and South Ruislip which selects its candidate in coming weeks, or Cecil Parkinson’s old seat Hertsmere in Hertfordshire (2010 Tory majority 17,605).

John Randall, the outgoing Tory MP for Uxbridge and South Ruislip, said Mr Johnson had visited the constituency “a lot”, adding: “Boris is a great politician and he will reach parts of the electorate I can’t reach.”

Ruth Lyon, chairman of the Hertsmere Conservative Association, said: “If he did want to stand in Hertsmere I think most of our members would welcome it.”

Once he is selected in a safe Tory seat, Mr Johnson will be free to fight the next election campaign, and his road to the leadership becomes more complicated.

Mr Johnson will be in the big league now, eyeing his rivals for the crown like George Osborne and Theresa May warily.

A number of scenarios will play out. If the Conservatives win the election, Mr Cameron has said that he wants Mr Johnson in his Cabinet.

Business, transport, or a new form of infrastructure brief are likely contenders; or alternatively a roaming brief as minister without portfolio.

But Mr Johnson has made clear that will not put himself forward to be a minister until after his term as Mayor in May 2016.

That would give him little time to get ministerial experience before Mr Cameron might step down as Tory leader and Prime Minister, rumoured to be in 2018.

But if the Tories lose the election, Mr Cameron is widely expected to quit as Tory leader, opening the way for a leadership election in October next year against Mrs May, Mr Osborne and most likely a candidate from the 2010 intake.

That too could be complicated for Mr Johnson, who will try to campaign to be leader at the same time as his full time day job as London Mayor.

It is also possible that Mr Johnson decides not to stand because he might not be attracted by four years as Opposition leader – he spoke on Wednesday of his experience in the last decade of the “grinding indignity” of Opposition.

And the final scenario sees the Conservatives emerge with the most votes but no outright majority.

This could see Mr Cameron attempt to run a minority Tory Government, which could collapse at any moment precipitating another general election.

Or there could be another Coalition Government – although it is possible that backbench Conservative MPs will not want to stomach this.

It is also hard to see Mr Johnson working with the Liberal Democrats and particularly its leader Nick Clegg, who he has personally attacked regularly.

He was previously sacked from his role as Shadow Arts Minister for lying to the then party leader Michael Howard about an extra-marital affair, dismissing the allegations as an “inverted pyramid of piffle”.

It would mark an extraordinary journey for a man who came to prominence hosting topical news programmes such as Have I Got News For You.

Take a look back at some of the highlights – and occasional lowlights – of his career to date.

I think the objective of the briefing was to stimulate an article on roughly the lines that you see before you: roughly, that is, but not exactly. Sometimes you begin a discussion with a general prejudice to agree with someone – and then, as the conversation goes on, you find yourself prey to misgivings.

The young woman from Dignity in Dying was charming and persuasive, but as we talked about the exact terms of the Bill, I started to worry that it might indeed pave the way for something unintended – something ugly and distressing: not exactly a culture of death, but a world in which it was simply too easy to opt for this state-approved self-extinction.

The problem lies in the first couple of paragraphs, where the Bill defines those who would be eligible to take the “medicine” (a euphemism if ever there was one) or to have it administered to them. They must be someone who has a terminal illness and who “as a consequence of that terminal illness is reasonably expected to die within six months”. Now I am sure that infinite care has gone into the drafting of that phrase, but it surely encompasses a potentially very wide group of people.

Many hundreds of thousands of people, old and young, find themselves living on with terminal illnesses – cancers and other afflictions – for a very long time; and obviously a great many will prefer life to the alternative. It goes without saying that many are capable of taking continuing interest and pleasure in their lives. But it is also true of those people that their conditions could deteriorate quite quickly – and that they could die within a relatively short space of time.

In other words, there are a large number of people whose deaths would not come as a huge surprise to a doctor – and who could therefore be described as people who could “reasonably be expected to die within six months”. I would like to see this language tightened up, so that the category of those who might be eligible for the “medicine” is not as broad as it currently seems. Surely it cannot be beyond the skills of the parliamentary draftsmen to amend the phrase slightly, so that assisted dying could be provided only to those who “could not reasonably be expected to survive more than six months”.

You may say there is not much difference between the two categories. I think the difference is very significant. It is the difference between the strong possibility of death within six months, and the overwhelming probability of death within six months. It is the difference between the category of those who could very well be claimed by the Reaper at any time within the next six months, and the category of those who are clearly not going to survive much longer. This change would restrict the number of those eligible for assisted dying – though obviously it would still greatly expand the possibilities for alleviating suffering at the end of our lives.

I think it right to be cautious, because we are proposing to make a very big change in our approach to death and dying: giving people much more of a right to choose when to die, and above all giving the state an obligation to help us consciously to do away with ourselves, if certain conditions are satisfied.

I am sure it is a change the public broadly supports, and one whose time has come. But life is precious and our psychology fragile. Those suffering terminal illnesses can easily find themselves under pressure – external or internal – to make decisions from which, obviously, there is no going back.

If we are going to take this step, we should make it a small one, and see how it goes. I would like to see the Falconer Bill apply not to all those who might well die in the next six months, but only to those whose lives are overwhelmingly likely to be very near the end.

However, if he did so, he would have to combine being an MP with continuing as Mayor of London until his term expires in May 2016.

If he returns to the Commons he will want to find a seat which is in easy rich of central London. The seat in Hertsmere would offer just that for Mr Johnson.

Mr Clappison, who has been an MP for 23 years, is a well-regarded member of the Home Affairs select committee and the European Scrutiny committee.

In a letter to Mr Cameron, Mr Clappison said: “It is difficult letting go but I do feel now is the time to move on. There is a world elsewhere.”

Separately, Mike Weatherley, MP for Hove and Portslade and one of the intake at the 2010 general election, also said he was quitting as an MP next year.

In his letter to the Prime Minister, Mr Weatherley said he was leaving for personal reasons, stating that “beating cancer two years ago”.

He said: “Beating cancer two years ago has led me to review what I want for the future. It has been the toughest decision of my life but I do feel that now is the time to move on.”

To celebrate the first anniversary of Ask Boris, Boris Johnson’s monthly LBC show, the Mayor and presenter Nick Ferrari broadcast from high above the Thames on the Emirates Airline.

“It is cheap as chips,” Mr Johson responded when asked how much a cash single fare would set a passenger back.

“I think it’s very good value for money, this is the most beautiful view in the whole of the city, I think it’s absolutely stupefying. Earth hath not anything to show more fair,” he said before pointing out landmarks including “the Walkie Talkie with its Aztec death ray.”

A single cash fare is £4.40.

The benefits were obvious. If we could liberate the shale gas beneath us, then we would lessen our dependence on supplies from Russia and the Middle East. We would be able to cut fuel prices for hard-pressed consumers. We would be able to cut energy costs for British industry, so accelerating that exciting process of “reshoring” manufacturing, and rediscovering our identity as one of the world’s great centres of high-value production. We would be able to use the gas – much cleaner than coal – while we sorted out our chaotic energy supply. We would be able to invest in some serious nuclear power plants, and some proper renewable energy, other than those ludicrous wind farms. Follow solarkraft for more information.  All we needed to do, as everyone kept saying in those dim, distant days, was to “get fracking”.

What is the problem? Well, of course, it is the bureaucracy, the protests, the general bad vibe that is associated with fracking, and the hostility of anyone who thinks they might be living in the vicinity of a well.

But there is one decisive difference between Britain and America – and it has nothing to do with our respective cultures of enterprise. Nor is it really about bureaucracy. As anyone who has lived in America can testify, it is just about the most heavily regulated country on earth. No: the big difference is that in America the landholder has the rights to all the minerals beneath – all the way to the centre of the earth. Here, the rights to gas and oil are vested with the Crown.

To understand this anomaly, you need to go back 100 years to the time when UK legislators were first considering what was then viewed as the remote possibility that the country might be endowed with significant reserves of oil and gas. They had seen what was happening in America, and – as today – there was a vague attempt to get at Britain’s resources. The trouble was that nobody had found much, and when they had found something there was an instant wrangle between adjacent property-owners. One owner would claim that the other had siphoned off the oil beneath his property, and that he had not been properly compensated. The result was that no one could be bothered even to try to get at British oil – if there was any.

So, in 1919, the then minister for munitions, Frederick Kellaway, decided that “it would be an almost insoluble problem to devise any equitable scheme for the allocation of royalties as between neighbouring landowners, owing to the relatively small size of estates in this country, and the impossibility of determining from what point in a petroliferous area the supply tapped by any boring is actually drawn”. So he came up with what he thought was the equitable solution of assigning all such rights to the Crown.

That was the origin of the 1934 Petroleum (Production) Act, which forms the basis of the present law. It was intended, paradoxically, to encourage extraction at a time when it seemed a fairly hopeless venture. Today, when we have actually found the stuff – and when it is much easier to tell precisely where and beneath whose land it is located – the measure is causing paralysis. The result is that no landowner, large or small, has any automatic commercial interest in the discovery of shale gas beneath their property. No wonder the shires are in revolt against fracking. It is no surprise that everyone is a Numby when they are told that what is under their back yard is not theirs, but belongs to the Queen!

It is time for a simple and profoundly Conservative – not to say Thatcherian – change to the law. People have the rights to any diamonds or platinum or uranium they may excavate beneath their homes. Why shouldn’t they have the right to the one mineral that may – it now transpires – exist in glorious abundance? Her Majesty the Queen is absolutely magnificent in every respect, but she has oodles of land already. It is time to give the Numbies a stake in the matter. Give the British people their mineral rights, and get fracking at last.