Battle to win floating votes

The two leading candidates were contacting hundreds of thousands of voters in the belief that the contest remains on a knife edge.

Mr Livingstone's Labour campaign sent out one million postcards with the slogan "Don't Vote for A Joke - Vote for London" while his Tory rival was emailing 300,000 undecided voters saying that victory for Labour would allow the Prime Minister and the Mayor to "think they can get away with anything and never be held to account".

The most crucial battle is over the second preference votes given by people who use their first choice to back Mr Paddick. It exploded into the open at a Sky News debate last night .

Mr Paddick has little chance of becoming mayor but his endorsement with hundreds of thousands of votes could swing the election either way.

He has so far refused to name either of his main rivals as the "next best" option, claiming that Londoners should be allowed to decide for themselves. In the final live debate, Mr Livingstone said he would support the Liberal Democrat over Mr Johnson "without a moment's doubt".

He asked Mr Paddick: "Why can't you express a choice for the lesser evil because that's what all politicians do?"

Mr Johnson accused the Mayor of a "sickening orgy" of chasing his Lib-Dem opponent's vote but moments later added: "I think Brian has some wonderful things to say."

Mr Paddick, meanwhile, urged voters to pick him first and then vote tactically with their second preference to keep out whichever of the two others they did not want as mayor.

Mr Johnson's attempt to link the race for City Hall to difficulties for Labour nationally is a clear change of tack. His "personal message to Londoners" thanks all those who have supported his campaign and claims that his key pledges - fighting crime, protecting green spaces, spending taxpayers' money wisely, easing congestion and improving public transport, and properly overseeing planning and housing - have "struck a chord" with voters.

But Mr Johnson warned that the election remained finely balanced despite polls showing him in the lead and claimed that the next few days would be "critical".

His email was addressed to "those who may still be uncertain whether to vote, or for whom" and went on to state that "the decisions the Mayor takes can make life better ... OR ... as under the current Labour Mayor, worse".

"If Labour win on Thursday, Gordon Brown and Ken Livingstone will think they can get away with anything and never be held to account," it said. Mr Livingstone has continued to out-poll his own party by distancing himself from it, but today's message tries to link the Mayor and Downing Street.

The Mayor started the final 48 hours of campaigning by targeting Londoners' worries that Mr Johnson is not serious enough for the job. The campaign, which will see the postcards sent to every area apart from Tory heartlands, will also use the slogan in newspaper adverts and on billboards.

Although a new YouGov/Evening Standard poll gave the Tory candidate a healthy lead this week, it also confirmed that many electors have doubts. Some 42 per cent believe he lacks the seriousness required to make an effective Mayor.