Boris Johnson is the bookies' favourite to be the next Prime Minister

The Mayor of London has overtaken George Osborne for the first time

Mayor of London Boris Johnson (JUSTIN TALLIS/AFP/Getty Images)

Boris Johnson is now the bookies’ favourite to succeed David Cameron as Prime Minister for the first time.

Mr Johnson yesterday announced that he would back the campaign for Britain to leave the European Union – triggering a surge of publicity.

The bookies Paddy Power this morning named Mr Johnson as the new favourite for next Prime Minister this morning, down from 3/1 to 5/2.

This is the first time the Chancellor George Osborne – widely believed to be Mr Cameron’s preferred successor – has not led under the metric.

Mr Cameron has said he will step down before the next election – meaning the person who replaces him as Tory leader will become Prime Minister, at least briefly.

There have also been suggestions that Mr Cameron might step down if his “in” camp loses the EU referendum. If this were to happen Britain might have a new PM as early as this June.

Conservative leadership nominees are voted on by the party’s MPs until only two remain, at which point the final pair are voted on by the party’s entire membership.

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.

Boris Johnson met with a rude gesture while out cycling (Getty Images)

Narrow favourite was former defence secretary Liam Fox, from the hard right wing of the party. Mr Fox said today he would “never say never” to standing for the position.

Mr Fox is just ahead of Theresa May in second place, Mr Johnson in third; followed by Mr Osborne and Sajid Javid, the Business Secretary.

“The direction of events is clear … those members associated with Brexit – such as Chris Grayling and Priti Patel – are seeing their ratings rise sharply if their previous standing in it was relatively low,” wrote Paul Goodman, the website’s editor at the time.

Not all assessments of Mr Johnson’s move put him in poll position for the leadership, however. His 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.

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