Mayor Ken Livingstone has rejected suggestions London taxpayers will automatically be forced to cover any cost over-runs for the 2012 Olympics.

Londoners will pay an addition sum on the council tax precept for the next 10 years to help fund the Games.

Band D homeowners, which form the single largest group across the eight bands, will pay an extra £20 on their precept, under a deal agreed by the Mayor of London and the government.

Members of the London Assembly yesterday asked Mr Livingstone to guarantee residents would not face rises in the precept if the Olympics went over the present £1.5 billion budget.

"I see no value in starting an argument with the government about a hypothetical situation," he said yesterday, appearing at mayor's question time in front of the assembly.

Mr Livingstone said he would work hard to make sure Londoners did not bear the brunt of any additional costs, and that the council precept was already high enough.

"The Government has a policy that, if there is an over-run, we will discuss it," he said.

Olympic chiefs denied a string of media reports last month that the cost of hosting the 2012 Games was to double.

Dee Doocey, the Liberal Democrat Olympics spokesperson, said Mr Livingstone should be offering a clear guarantee to protect Londoners from bearing the brunt of cost over-runs.

The Lib Dems say the precept would have to double, for example, if the cost of the Olympics increased by 20 per cent.

Any extra costs are likely to be met by the National Lottery or the Mayor of London.