Boris Johnson's lead over Ken Livingstone is slim enough for the Labour candidate to pull off a surprise victory on Thursday, a new poll suggests.
The poll comes amid dangerously poor results for the Conservatives, which show Ukip could do major damage to the governing party during this week's local elections.
The latest YouGov poll for the Standard puts Boris on 44% to Ken's 41%, with Liberal Democrat candidate Brian Paddick on six per cent. Once second preference votes are allocated, Boris leads by four per cent on 52% to Ken's 48%.
The margin is slim enough to constitute a statistical dead heat. Both camps are likely to welcome the result as a motivator to get their supporters to the polling stations on Thursday.
Meanwhile, Green candidate Jenny Jones, Ukip candidate Lawrence Webb and independent Siobhan Benita are all scrambling for fourth place on three points.
The YouGov poll suggests the race is closer than results from ComRes or Survation, both of which gave Boris a much stronger lead.
It comes as Downing Street contemplates a bloodbath in the local elections, which take place alongside the mayoral vote this week.
A YouGov poll for the Sunday Times puts the Tories on 29% - the first time they have slipped below the 30% mark.
The drop comes amid a spectacular performance from Ukip, which is polling at ten per cent.
The Conservatives have lost nearly one fifth of their support since the Budget five weeks ago.
Amid an endless series of bad headlines, David Cameron's personal popularity has also plummeted.
His rating has fallen from minus five to minus 31 since the Budget.
Only Gordon Brown can match that level of decline, after voters turned their back on him when he called off a potential election in 2008.