Ken Livingstone is losing ground to Boris Johnson in the race for City Hall, with just under a month of campaigning to go.
A ComRes poll for the Evening Standard, London Tonight and LBC put Boris on 50% to Ken's 46%, on a second-round vote.
The result is a major boost to the incumbent, who had been two points the Labour challenger in the last Comres survey.
The poll suggests 46% of Londoners would back Boris in the first round of voting and 41% would back Ken. Six per cent would vote for Liberal Democrat candidate Brian Paddick and four per cent would vote for Green candidate Jenny Jones.
The decline in Ken's poll performance comes after a difficult period for the candidate, as he struggled to move on from allegations over his tax affairs.
During a radio interview, the Labour candidate accused the mayor of similar tax avoidance tactics leading to a shouting match in a lift where Boris branded him "a f**king liar".
Boris appears to have been the main beneficiary of the row with 51% of respondents saying they were unlikely to vote for Ken afterwards, compared to 44% beforehand.
Ken continues to dominate in inner London, with 58% support compared to Boris' 38%, but even here Ken's support has been slashed by ten per cent since January.
In outer London, where Boris dominates, he has 57% support, compared to 39% for his rivals.
Tellingly, Boris has a higher level of support among voters than his own party, while Ken has slightly less.