Ken Livingstone has a narrow lead over rival Boris Johnson in the race to become mayor of London, a new poll has revealed.

A survey carried out by IPSOS-MORI, commissioned by the trade union UNISON, showed Mr Livingstone securing 41 per cent of respondents' votes, with Boris Johnson breathing down his neck at 40 per cent.

Lib dem candidate Brian Paddick had 14 per cent while Green Party candidate Sian Berry had 5 per cent.

The poll is the first to show the current mayor, who is seeking a third consecutive term in office, to be ahead of Johnson who had a 13 point lead over Livingstone in a YouGov poll for the Evening Standard newspaper.

This week has seen Mr Livingstone's campaign focused on extending the reach of the transport system. The mayor visited Muswell Hill on Thursday (April 10) where he outlined his plans for London's bus system.

"The greatest testament to the improvements made to London's bus services is that after years of decline and decay Londoners have gone back to using the buses in their huge numbers. And despite a 40 per cent increase already, passenger numbers are still increasing year on year," he said.

Livingstone also criticized Boris Johnson's bus plans, claiming they had descended from "farce to total chaos".

He said: "He (Johnson) cannot explain how much his flagship policy would cost, although independent experts calculate it would be at least £114m. This would mean Londoners paying £2 extra on a weekly bus fare."