An eve-of-poll survey for the Evening Standard has indicated that Boris Johnson is likely to be re-elected as Mayor of London.
According to the YouGov poll Conservative Boris Johnson is leading his Labour rival Ken Livingstone by 53 per cent to 47 per cent.
The hotly contested battle for the second most powerful job in politics is held under a supplementary vote system and uses a series of rounds to establish a winner.
The YouGov poll suggested as many as two in ten Labour supporters may have decided not to back Ken Livingston despite calls from key party figures to 'hold their noses' and vote for the Labour candidate.
In response to the poll, a spokesman for Mr Livingstone said, "Today, Londoners can vote Labour to cut their fares and save themselves on average £1,000, and in doing so ensure that the Conservative Party is not rewarded. Every Labour voter must turn out today or the Tories will get away with it - they will carry on with policies that have led to recession, fare rises and police cuts."
There were signs also today that turnout may be lower than expected as voters made their way to polling booths under grey skies and rain.
At a Streatham polling station, in south London, 115 voted in the first three hours.
Predictions suggest the final turnout could be as low as 38 per cent, based on a formula that takes into account the fact that half those voting tend to do so after 5pm and allowing for postal votes.
Boris Johnson made a final bid for votes with a message on YouTube on what he said was "a critical day in the life or our city".
Despite the possibility of a win for the Conservatives in London, elsewhere the predictions for David Cameron are gloomier with his party expected to perform badly in the local elections.
Conservatives and Liberal Democrats are braced to lose hundreds of the around 5,000 seats at stake on 181 local councils across England, Scotland and Wales.
Many councils will begin counting the votes this evening with results expected overnight and tomorrow.
However, counting of the votes for the London Mayor contest will not begin until tomorrow morning.
The YouGov poll surveyed 2,119 London voters between Monday and Wednesday.