The race between Ken Livingstone and Boris Johnson is "too close to call" an influential study found today.
Many Londoners will not make up their minds until the final days of the campaign, meaning the result could go either way.
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 the space of 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.
