Former Olympic Delivery Authority chairman Jack Lemley claims work has been obstructed by land contamination problems and political meddling.

But acting chair Sir Roy McNulty said the ODA board "does not accept the version of events that he has set out".

McNulty added that there would soon be a "robust, realistic budget" for 2012.

The budget would be settled after talks with the Treasury and the Department for Culture, Media and Sport (DCMS) end in the new year.

Lemley, who ran the group which designed the Channel Tunnel, quit as ODA chairman in October just seven months into a four-year contract.

He claimed Olympics Minister Tessa Jowell and London Mayor Ken Livingstone repeatedly ignored his warnings that the Games budget was spiralling out of control.

The Stratford site is complex and much of the land will need remediating before construction Acting ODA chair Sir Roy McNulty

Last month, Jowell told MPs the cost of London's 2012 Olympic Park has soared by �900m, up from �2.375bn to �3.3bn.

But McNulty said: "The ODA board would also like to make it clear that we have seen no signs of unreasonable pressure (from the government) to cut costs.

"The only reductions made to the May figures have been those which the ODA and LOCOG (London Organising Committee of the Olympic Games) have identified as achievable.

"As has been widely acknowledged, the financing requirement for the overall programme will be higher for reasons that have been explained.

"Mr Lemley will know that costs have not been rising exponentially."

Lemley also claimed fears over the chemical contamination of the Olympic site in Stratford were dismissed because the Government only wanted to hear "good news".

But McNulty added: "He has, for example, presented the issue of land remediation of the Olympic Park site as though this were a new discovery for the project. It is not.

"The Stratford site is complex and much of the land will need remediating before construction.

"This was known from the very start of the bid process."