What most excites Tories about Boris Johnson's coming return to parliament is the belief that, as leader, he will be able to deliver what David Cameron has not: a Conservative majority. The Mayor defied political gravity to twice win election in Labour-voting London leading him to be dubbed "the Heinkeken Tory": the man who reaches parts of the electorate that others cannot.

Today's YouGov/Sunday Times poll shows that he is the public's top choice to be the next Conservative leader, backed by 30 per cent, compared to 16 per cent for Theresa May, 7 per cent for George Osborne and 3 per cent for Michael Gove. But despite this, when asked how they would vote if Boris led the Tories (and if Ed Miliband and Nick Clegg still led their parties), the Tories' share increases by just one percentage point from 33 per cent to 34 per cent.

Although the Mayor would prove successful at attracting Ukip voters, with 20 per cent of those who currently support the party backing the Tories, he would have little effect on Conservative support among current Labour and Lib Dem voters. Worse, he would actually repel current Tory voters, with 92 per cent backing a Johnson-led Conservative Party compared to 97 per cent for a Cameron-led one.

This is partly because many simply don't think he's up to the job of prime minister, with 36 per cent saying he is and 43 per cent saying he is not. By contrast, a majority of voters (52 per cent) believe Cameron is up to the job, with 37 per cent saying he is not. This is a reminder that the Tories already have a relatively popular leader, who currently outpolls his party by eight points (41 per cent to 33 per cent). That Cameron supporters don't automatically become Conservative supporters is a reflection of the Tories' enduring brand problems.

While it's always wise to treat hypothetical polls with caution, today's poll does suggest that the election of Boris as leader won't alone be enough to boost the Tories unless they undergo more fundamental change.