Boris ‘would be a vote winner for Tories’ at general election

A survey conducted in Uxbridge and South Ruislip after the London Mayor finally confirmed his desire to return to the Commons showed he could significantly boost Tory support.

But a third of those who backed the colourful Conservative also said he should quit City Hall if elected – despite Mr Johnson insisting he could see out the final year of his term.

The poll of 1,000 people in the seat being vacated by former deputy chief whip John Randall was carried out for Tory donor and former party vice chairman Lord Ashcroft.

Mr Randall held the seat with a majority of 11,216 at the last general election, making it a plum target for Mr Johnson, who has remained tight lipped about his preference.

The deadline for applications to the local party is August 28.

Lord Ashcroft said that when asked generally about their 2015 intentions, 42 per cent of voters said Conservative, 28 per cent Labour and 19 per cent Ukip.

But when they were faced with the prospect of Mr Johnson being the candidate, the Tory share rose to 52 per cent, with Labour down five to 23 per cent and Ukip down three to 16 per cent.

The peer accepted that the figures would be different if other parties put up a high-profile challenger but said Mr Johnson had a “unique ability both to galvanise Tories and appeal to supporters of other parties”.

More than half (53 per cent) said he was “different to most politicians, and in a good way” compared with only seven per cent saying he was different “but in a bad way”.

Voters overall were evenly split on whether he should continue as mayor until 2016 if elected as an MP.

Mr Johnson last week declared his intention to stand for Parliament at next year’s general election, finally putting an end to months of speculation about his ambitions for a return to Westminster.

It immediately raised the prospect that he could emerge as a rival for the Conservative leadership if the party is defeated – or again fails to secure an overall Commons majority – at the polls.