David Cameron openly ridicules Boris Johnson over leaving EU and attacks his leadership ambitions

The PM hinted that his would-be successor had an 'agenda' for leaving the EU

David Cameron heavily criticised Mr Johnson's position (Crown copyright)

David Cameron has publicly hit out at Boris Johnson’s backing for the EU “leave” campaign, hinting that the Mayor of London may have made the decision with his career in mind.

On a day in which media coverage of the EU referendum has been dominated by the decision of Mr Johnson to back Brexit, Mr Cameron used a statement in the House of Commons to hit out at his fellow Tory MP.

“I am not standing for re-election,” he Mr Cameron said, apparently a veiled reference to Mr Johnson’s leadership ambitions.

“I have no other agenda than what is best for our country. I'm standing here telling you what I think.”

Mr Cameron also ridiculed Mr Johnson’s ambiguous apparent suggestion in a recent newspaper column that voting to leave could lead to a better deal for Britain in Europe.

“We should also be clear that this is a final decision,” he told MPs, arguing that a second renegotiation was “not on the ballot paper”.

“I won’t dwell on the irony that some people who want to vote to leave apparently want to use a leave vote to remain. But such an approach also ignores more profound points about democracy, diplomacy and legality.”

The PM said that the idea that other EU states would give Britain more changes to EU structures was “for the birds”.

Mr Cameron was egged on his his criticism by opposition MPs. Liberal Democrat leader Tim Farron urged the PM was thwart the “blonde ambition behind him” – reference to Mr Johnson’s trademark hair.

Former Labour leadership Yvette Cooper also asked the PM to condemn Mr Johnson’s claim that Britain could easily negotiate new trade deals outside the EU because it "used to run the biggest empire in the world".

Mr Johnson was reportedly seen to shout “rubbish” from the backbenches during Mr Cameron’s speech and was described by observers as looking “very cross”.

He had written in his Daily Telegraph column this morning that voting to leave the EU would be seen as an opportunity by Brussels “to strike a new and harmonious relationship with Britain” – interpreted by some as the possibility of remaining in the EU.

Mr Cameron made clear that that believed remaining in the EU after a referendum was not an option and that to do so would be undemocratic.

The assessment that Mr Johnson’s move put him in poll position for the Conservative leadership was not shared by all. The MP’s father, Stanley Johnson, this morning said his son had “thrown away” a position in Mr Cameron’s Cabinet by making the pledge to back “out”.

“I can’t think of a more career ending move than to do what he did yesterday,” the former MEP told BBC Radio 4’s Today programme.

The Mayor of London, whose term of office ends in May, is however now the bookies’ favourite to replace Mr Cameron as Conservative leader for the first time, according to PaddyPower.

A recent survey of a panel of members by the website ConservativeHome found a close race in which eurosceptic candidates had gained at the expense of those in favour of remaining in the EU.

This weekend Mr Cameron announced that Britain would hold its in-out referendum on membership of the European Union on 23 June of this year.

The vote follows a renegotiation of the terms of membership of the EU by the Prime Minister.

The PM has granted his Cabinet colleagues permission to campaign on opposite sides of the referendum – an unusual approach to government collective responsibility.