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
