Boris Johnson’s road to Conservative leader and 10 Downing Street

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.