Labour's big guns were out canvassing for Ken Livingstone today amid fears that losing London could have national repercussions.
David Miliband, bookmakers' favourite to succeed Gordon Brown, warned voters that Boris Johnson was not serious enough to run the capital.
The Foreign Secretary said the Mayor had put London at the top of international leagues and his Conservative rival would "fritter that away" if elected.
Campaigning in Palmers Green, he said: "Boris Johnson may be bored on the backbenches but London is a great international city not a toy.
"London needs strong leadership from Ken Livingstone, not entertainment for Boris Johnson. It has taken 10 years to put London at the top of the international leagues. It is vital that the next 10 years do not fritter that away."
His backing came as Labour figures admitted privately that losing London could be damaging for the Prime Minister. Publicly, they insist the race is not a referendum on the Government but about issues affecting the capital.
Mr Miliband told voters that in "each and every one" of the 20 countries he had visited as Foreign Secretary the capital was a source of admiration and envy.
"Over the last 10 years it has come to stand for something simple: the one to beat. In terms of its economy, its culture, its people, its daring, its resilience, it is world class. That doesn't happen by accident; it takes leadership," he said.
"Ken Livingstone has provided the vision that has helped London reclaim its place as the number one international city. And critically London is moving forward not standing still. The best thing about Ken is that he is restless for more change not satisfied with the status quo."
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Minister for London Tessa Jowell was out on the stump for Mr Livingstonein north London yesterday. Meanwhile, an influential study found today that the contest between Mr Livingstone and Mr Johnson is "too close to call".
The London Communications Agency report suggested the result would depend on second preference votes.
Crucially, it could turn on whether Lib-Dem Brian Paddick endorses either of his main rivals, which he has so far refused to do.
It comes after a YouGov poll for the Evening Standard showed Mr Johnson's lead over his Labour rival had halved in a week. "Both candidates can win," the study said.
"Ken needs to explain how his record to date has benefited London and adopt a more consensual tone of voice. He needs to remind people of the cheeky chappy Londoners liked in the first place. Boris needs to convince voters that he is up to the job as well as just being likeable." It concluded that only the Labour or Tory candidates stood a realistic chance of winning.
London local government expert Tony Travers said: "There is some evidence there is a small lead for one candidate but this is so small you have to call it a 'to the wire' contest."
The LCA said there would be 10 new faces on the London Assembly although its overall make-up would remain almost the same - nine Tories, seven Labour, five Lib-Dems, with the Green party up one to three seats and Ukip gaining one seat.
