Conservative London mayoral candidate Boris Johnson has been forced on to the defensive over his plans to install a new fleet of Routemaster buses for the city.
Mr Johnson was overheard telling a Labour supporter during a walkabout that the total cost of the Routemaster policy could cost around £100 million more than he had initially suspected.
Labour's London minister Tessa Jowell jumped to the attack in response to the news, saying Mr Johnson had been "caught out" and forced to admit his plans on buses were not funded.
"Even Boris Johnson doesn't believe the figures he has been opportunistically touting around for weeks," she commented.
"What Londoners want are real solutions to the problems they face in their daily lives, which Ken Livingstone is offering. Boris Johnson is finding out that unfunded pledges won't wash with Londoners."
The additional £100 million would cost Londoners an extra £2 every week for the weekly bus pass, she added.
Mr Johnson himself insisted the new generation of Routemasters he is backing will cost his previous figure of £8 million.
"I stick by our figure of £8 million as the cost of conductors on roughly 350 new generation Routemasters," he said on BBC1's London News programme yesterday evening.
But he admitted: "It is true that if you have three conductors then you have three shifts and it goes up."
He has previously suggested Routemasters may even prove cheaper on a bus-by-bus basis than the bendy buses currently used on London's busiest routes.
Liberal Democrat candidate Brian Paddick continues to back his policy of using tram lines.
Speaking on the same programme, he insisted his proposal was financially sound after it was suggested the cost of building the infrastructure for running the trams would more than cancel out the financial benefits.
"On a busy route trams cost less than even bendy buses. I am the only candidate with a business degree so I do not need lectures on economics," he said.