Greens and Ken strike poll deal to beat Boris

In a surprise move that could alter the dynamics of the race, the Greens urged their supporters to give their second preferences to Mr Livingstone to prevent the Tory candidate taking City Hall.

Green mayoral candidate Sian Berry appeared alongside the Mayor at a nature reserve in Camden to declare that they had a "common cause" in attacking Mr Johnson's environmental credentials. In the previous mayoral election, the Greens told their supporters not to back any other candidate with second preferences.

But with Mr Johnson ahead in the polls, the London Green party decided at a meeting late on Monday night that backing Mr Livingstone was the best way of keeping the Tories out.

Ms Berry urged her backers to vote "Sian 1, Ken 2" on the ballot paper, while the Mayor asked his voters to give the Green candidate their second preferences.

Both Mr Livingstone and Ms Berry listed Mr Johnson's "green crimes", including his:

Opposition to plans for a £25 congestion charge for gas-guzzlers, as well as a 100 per cent discount for smaller cars.

Backing for George Bush's decision to boycott the Kyoto treaty on climate change.

Claims that the new low-emission zone for Greater London is "punitive" and "draconian".

Plans to allow motorcyclists to use bus lanes.

Opposition to road humps and traffic lights timed to favour pedestrians.

Only last month, the Greens decided against backing Mr Livingstone because they believed he needed to do more to boost the environment. But they have since been impressed by his policies for the election campaign and have changed their minds.

Ms Berry today moved to underline what she called "the double threat of a Tory monopoly over the GLA" - with Conservative Assembly members allowing a Mayor Johnson "to push whatever he wanted".

She said: "Boris Johnson has real contempt for Londoners. He hates that we celebrate each other's heritage; he hates that we try to pass on a healthy environment to our children; he hates that we look after our most vulnerable neighbours; and most of all he hates that we all expect to share in our city's financial success. And if he's elected he will do his best to destroy all of that.

"I cannot bear the idea of living under Boris Johnson. What I am urging Green supporters to do is this: vote Green on the Assembly, vote for me as Mayor, then use their second mayoral vote as an insurance policy to stop Boris Johnson by giving it to Ken Livingstone."

She said that the Mayor has needed the votes of the two Green Assembly members to pass his budget this term, requiring discussion, negotiation and compromise. Ms Berry herself is placed fourth on the Green list, and is expected to be elected to the Assembly if she loses out in the mayoral race.

Mr Livingstone said Mr Johnson had sided not with Londoners but with the big polluters such as Porsche cars, which has attempted legal action against the £25 charge plan.

"This common green agenda is at the heart of a modern, forward-looking agenda for London, which Sian and I both share and which Boris Johnson stands against," he said.

Blog: Do red and green go together?