Ken Livingstone and Boris Johnson went head to head over the real cost of bringing back Routemaster buses with conductors on London's streets.
The two rivals have clashed on the size of the bill passengers would face through fare rises.
Tory candidate Mr Johnson, whose first campaign pledge was to phase out bendy buses, claims the move would cost £8 million a year, which would be funded by cracking down on fare evasion.
But Mr Livingstone said the cost would be nearer £108million a year, which would mean the doubling of fares.
At a debate held by Transport Times, he said: "They're never going to be built, it's never going to happen."
Mr Johnson plans to hold a competition to design a new generation of Routemasters with conductors to gradually replace bendy buses. He said conductors would crack down on antisocial behaviour and make passengers feel safer — and would be paid for by the extra fares they collected.
"I stick by our £8million figure," he said. "It will be raised simply by ending £8million of fare evasion on bendy buses now."
Mr Livingstone said the cost would be much higher as the starting costs had to include tax and national insurance and the buses would be expensive to commission — even if a firm to manufacture them could be found because they were so dangerous.
