Ken Livingstone has come under pressure over the costs of redeveloping the London Olympics site.

At the mayor's monthly question time session in City Hall on Wednesday, Livingstone was pressed over comments from the chairman of his London Development Agency.

In a recent interview with the Financial Times, Mary Reilly said that in a "worse case scenario", the costs of buying and decontaminating the Stratford site could double to close to £1bn.

The comments appeared to contradict the mayor's previous assertion of confidence "that there will be no overspend".

Liberal Democrat assembly member Dee Doocey was set to ask Livingstone to guarantee that the capital's Band D council taxpayers will not be asked to pay more than £20 per year towards funding the Games.

"The Games offers a wonderful opportunity for London and indeed the whole nation," she said.

"From going around the capital, I know that Londoners are happy to pay their share of the staging costs, but they need firm evidence that these costs will be capped.

"Government ministers and the mayor of London saying 'trust us' is not that categorical assurance."

And Conservative Olympic spokesman Angie Bray said: "It is just months since we won the Games and already Ken Livingstone is backtracking on commitments not to run over budget.

"We may be looking at a doubling of costs. This is deeply alarming. If they can increase this much in such a short space of time, imagine how much they will soar by 2012.

"In the run up to winning the Games the mayor said he was 'confident' there would be no overruns.

"He doesn't seem so confident now. This is why we are so deeply concerned. He needs to explain himself."