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.