Boris Johnson is now 2/5 favourite - the shortest odds he has ever been - with William Hill, to become the Mayor of London.

Ken Livingstone's odds have drifted to the longest they have been since he assumed office, at 7/4, with Brian Paddick the 25/1 outsider.

"The momentum seems to have swung in Boris' direction and for the first time there seems to be a real belief amongst punters that it may just be beyond our Ken to win for a third time," said Hill's spokesman Graham Sharpe.

Paddy Power also has Johnson as their 2/5 market leader. Darren Haines, spokesman, said: "Punters have steadily been recognising there may be more substance to Boris' style than first thought and the 'Boris Bounce' shows no signs of falling yet.

"Betting has often proved a more accurate barometer in elections than many opinion polls in the past so, whilst there is a long way to go yet, Ken should beware the ides of March."

The bookie has already accepted over £50,000 on the outcome of the election, and is forecasting a massive six figures will be wagered over the coming weeks.