The initial budget for the 2012 London Olympics was "unrealistic and wishful thinking", a group of MPs said on Tuesday.

The Committee of Public Accounts said optimistic budgeting had resulted in costs soaring from 4 billion pounds during the bidding process to 9.3 billion in March 2007.

The Department for Culture, Media and Sport and the Olympic Delivery Authority (ODA) were criticised for omitting security, tax and contingency costs in the initial estimate and over-estimating the role of the private sector.

"It is now clear that the estimated cost at the time of the bid ... was entirely unrealistic," committee chairman Edward Leigh said in a statement.

"It ignored foreseeable major factors such as contingency provision, tax obligations, and policing and wider security requirements.

He went on to say: "At the same time, the estimate of the extent to which the private sector would contribute funding towards the Games has proved little more than wishful thinking."

Despite Treasury guidance to include contingency provision, it was omitted in the initial estimate, the committee said. When it was included in March 2007, it amounted to 2.75 billion pounds.

Tax liabilities of 836 million pounds also needed to be added to the 2007 bill.

The initial budget also left out the 600 million pounds needed for policing and security.

The public sector was initially supposed to contribute 3.4 billion pounds, with the private sector providing a further 738 million.

But the private sector contribution has been revised downwards to 165 million pounds, less than 2 percent of the total funding, the committee said.

The public funding requirement has risen by 5.9 billion pounds - to be paid by the Exchequer and the National Lottery.

The committee also criticised the lack of detail about the Games' legacy. Failure to provide such information could result in the public losing confidence in the government's ability to deliver the Games within the new higher budget, the MPs added.

ODA Chief Executive David Higgins said: "We believe we have a realistic budget and adequate contingency and are confident that we can deliver within it."

Olympics Minister Tessa Jowell said much of the MPs' report focused on the early stages of the budget planning, much of which took place three or four years ago.

"A lot has changed since then," she said in a statement.

She said progress had been made on the Olympic site, and work was continuing to budget and deadline.

The funding package announced in March 2007 "remains unchanged and robust".