Claims that the methodology of today's poll are flawed were rejected by YouGov.

A spokeswoman for Mayor Ken Livingstone's campaign claimed "this YouGov polling is wildly at variance with normal polling".

"We were prepared to treat the last YouGov poll as merely a rogue one but these results show that there is something clearly systematically wrong with its methodology," she said. "On the day before the last Mayoral election YouGov put Ken Livingstone just two per cent ahead of the Conservative Steve Norris, even though Livingstone actually won by nearly 11 per cent and this YouGov poll is of the same type."

But Peter Kellner, president of the polling firm, said it robustly stood by its findings and added that it had correctly predicted the previous election.

"In our final poll for the Evening Standard in 2004, our final figures for the Livingstone-Norris run-off, among all those giving a voting intention (that is, the same basis on which we report today's figures) were: Livingstone 55 per cent, Norris 45 per cent.

"The actual result was: 55.4 per cent /44.6 per cent."

He added: "In the light of these figures, we are unafraid of any comparison between online and telephone samples in London."

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