Boris Johnson has surged to become Tory activists’ favourite to lead the Conservative party – after backing a Leave vote in the European Union referendum.

A regular survey of party members by the ConservativeHome website saw the Mayor of London surge into a double-digit first-place lead.

Just a month ago Mr Johnson was in third place behind former defence secretary Liam Fox and Home Secretary Theresa May.

David Cameron has said he will step down at the next general election; a number of MPs are vying to succeed him.

There has even been some speculation that Mr Cameron might resign in the wake of an “out” vote in the EU referendum – which is to be held on 23 June.

Mr Johnson is now favoured by a third (33 per cent) of Tory activists on the website’s panel, up by 14 points on the previous month’s survey.

He is followed by the equally eurosceptic Michael Gove, who has also surged into second place, favoured by 20 per cent. Behind him is Liam Fox, supported by 18 per cent.

George Osborne’s fortunes have waned since the start of the EU referendum campaign – he is now only supported by 11 per cent.

Theresa May, who like the Chancellor favours remaining in the EU, has seen a similar drop and is on 10 per cent.

Support from party members appears to be strongly correlated with support for leaving the European Union.

“Essentially, it looks as though Boris, Gove and Fox are sharing between them the backing of the 70 per cent or so of Party members who, according to our last survey, will either vote Leave or say that they are likely to do so,” wrote Paul Goodman, the website’s editor.

David Cameron hinted last month that he believes Mr Johnson may have backed a vote to leave the EU with his career in mind.

Mr Johnson’s father Stanley Johnson however dismissed the suggestion, arguing that the Mayor had sacrificed a place in Mr Cameron’s Cabinet with the move.

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A YouGov poll of Tory members released earlier this month found that activists believe Mr Johnson to be a better media performer, more in touch with ordinary people, and better at uniting the party than the Chancellor.

He is also thought of as having a better understanding of winning elections and being slightly more intelligent.