Ken Boris
Ken Livingstone is now ahead of Boris Johnson in the London mayoral election polls. REUTERS/Toby Melville

Labour's Ken Livingstone has overtaken Conservative incumbent Boris Johnson in the London mayoral polls ahead of May's election.

Livingstone had been trailing Johnson for months by as much as eight points.

According to the latest YouGov survey of 1,349 voting Londoners, 51 percent said they would be backing Livingstone in May compared to Johnson's 49 percent.

The pair have locked horns over Tube fares, with Boris overseeing a six percent rise in London travel costs in January.

Former mayor Livingstone, who was ousted by Johnson in 2008 elections but is running again, is promising to slash fares.

Critics say this pledge is fantasy as the numbers don't add up.

"It looks as his promise to cut fares on buses and the tube has made an impact. Among all Londoners, transport vies with crime as their top concern," said Peter Kellner, YouGov's president.

"Boris's new year problem is that he is seen as increasingly out of touch."

Kellner blames the "collapse in the 'Boris Labour' vote".

YouGov polling in June revealed that many traditional Labour voters in London were considering voting for Johnson over Livingstone.

"That said, it remains a significant achievement for Boris that he is still in the race. London is a Labour city," said Kellner.

"In the 2010 election, when Labour was trounced nationally, it still outpolled the Tories in London. It now enjoys a 15% lead.

"If everyone voted for mayor in line with their normal party preferences, Ken would be way out in front."