The YouGov survey suggested that Mr Johnson, the Tory candidate, was six percentage points ahead of Mr Livingstone, the Labour incumbent.
As reports came in of heavy voting in the Tory leaning suburbs of outer London, Labour sources were gloomy about Mr Livingstone's chances of holding on to power. The Conservatives said the result was "too close to call".
Neither the current Mayor nor Mr Johnson is expected to gain the 50 per cent share of the vote required for victory in the first round. At that point all the other candidates – including the Liberal Democrat, Brian Paddick, and Sian Berry of the Greens – will be eliminated, and their second preference votes redistributed between the two front-runners.
Last night London Elects, the body which organised the contests in the capital yesterday, launched an investigation after staff at a polling station in Lambeth spoilt ballot papers by writing on them.
The result is due to be declared between 7pm and 10pm today. But the closeness of the race – and the high possibility of a recount – means it could be announced even later.
In the past two months, opinion polls have consistently put Mr Johnson ahead in the race to be Mayor, with Mr Livingstone closing the gap as voting day approached.
The YouGov poll for London's Evening Standard yesterday gave Mr Johnson 43 per cent of first- preference votes, with Mr Livingstone on 36 per cent. When second votes were reallocated, Mr Johnson's share rose to 53 per cent with Mr Livingstone on 47 per cent, a margin of victory well within polling error.
Mr Livingstone has agreed an electoral pact with Ms Berry under which their supporters would pick the other as their second choice.
Mr Paddick, who received 13 per cent first preference support in the YouGov poll, has steadfastly refused to endorse either of the leading candidates. Whether Liberal Democrats opt for Mr Livingstone or Mr Johnson as their "least-worst" second choice could hold the key to the eventual outcome.
With the race on a knife-edge, the turnout and the last-minute decisions of the 14 per cent of Londoners who told pollsters they had not made up their minds could also have made a difference.
Turnout yesterday was predicted to be above the 37 per cent recorded in the previous election, in 2004. If Mr Johnson wins the mayoralty it will be a significant boost for David Cameron's Tories, and a disastrous setback for Gordon Brown.
The Henley MP gained momentum after being selected as the Tory candidate last autumn. He belied his own reputation for being gaffe-prone and was helped by a series of headlines about "cronyism" and alleged financial mismanagement among Mr Livingstone's advisers.
The Tories are expected to remain the largest party on the London Assembly and may increase their number of seats from nine to 10. The Greens are hoping to rise from two to three, while the British National Party could gain a seat.
A ComRes poll of business leaders showed a clear majority – 57 per cent – expected a Johnson victory. Almost three quarters (74 per cent) wanted him to win, against 18 per cent opting for the current Mayor.
Voters' voices
Penny Philpott, 47, Florist
I'm fairly happy with what Ken's done so far. I'm worried about Boris. He seems such an eccentric character. My second vote was Green. I believe in what they stand for and think we should take the environment more seriously.
Beth Ettinger, 30, Youth project manager
I quite like what Ken's done so far – the improvements in transport and his funding of youth projects. I like his Old Labour principles and I particularly did not want Boris. He's a bumbling fool. I also like the Greens' planning in principle.
Thomas Franks, 77, Retired
I never liked Ken anyway, he had too much going on, that business with the grant and Lee Jasper resigning. Boris is fresh – Ken, he doesn't seem to support the English. He celebrates Paddy's day, Muslim day, India day, but not St George's day.
Melody Sylvester, 36, TV producer
At the end of the day Ken's got results and has a proven track record. Boris is absolutely laughable. I just think he's a puppet for the Tory party. Ken's not perfect by any means but at the end of the day we're not voting for perfect people, we just want results.
Ola Alatrese, 46, Solicitor
You can't really fault Ken for his support of the black community. My second vote was for the Christian party because they want to uphold marriages and lower the divorce rate – I think that's really important for our communities.
Precious Obutu, 57, Nurse
Ken is the man for all races. Everything he's done in the last eight years proves that he is a people's man. He needs to continue his work, like all these bendy buses, helping kids get to school without paying. And he's best for crime also.
Tim Phillips, 41, Business writer
It's a stop the Boris vote. He doesn't even live in London. I've been to Henley and it's got nothing to do with London. The problems people face there have got nothing to do with real London problems. And for all Ken's faults, he's a real Londoner.
Ismail Kaya, 40, Take away worker
I know Ken to be a good person, he's helpful. I think he is the opposite of racism. He helps refugees and he's a good man. I am Kurdish and he supports us. I think he will be better than any other party.
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