Olympic chiefs were accused of understating the true cost of the 2012 Games yesterday as it emerged that the bill for the aquatics centre was already four times higher than the original bid price.

The Olympic Delivery Authority (ODA) announced that the contract, which was awarded to Balfour Beatty, had risen from the 2005 price of £75 million to £303 million.

Officials argued that the new budget includes a £60 million bridge that will merge into the roof of the centre, which has a 17,500 seating capacity, two 50-metre swimming pools and a diving pool. The price also incorporates VAT costs and 5 per cent construction inflation, which were not part of the original bid, they insisted.

The figures come after soaring costs at most of the Olympic venues and have led to renewed fears that the £9.3 billion 2012 budget will be exceeded. Jack Lemley, the former chairman of the ODA, gave warning that the bill was now likely to be £20 billion and he claimed that Ken Livingstone and Tessa Jowell had been understating the true costs.

The cost of the main stadium has jumped from £280 million to £496 million and the ODA admitted yesterday that the budget for the velopark had nearly trebled from £28 million to £80 million.

The ODA insisted that the £303 million figure was part of the baseline budget announced by the Government in December. The figures are considerably higher than the estimates of £150 million, which were discussed last autumn after a redesign to cut costs. There has already been concern that the lack of bidders for the big venues has allowed contractors to dictate prices.

Mr Lemley claimed in a newspaper interview yesterday that from very early on the ODA was working to an estimate of well over £12 billion, nearly four times the £3.3 billion original bid price. He said that he now believed the final cost would be £20 billion because of the state of the Olympic site, including polluted water and a plant discharging sewage.

“We will never be able to go really public with the full budget,” Mr Lemley said. “That was always suppressed and the mayor didn’t want any significant growth in these budgets because it would create a bigger tax.”

A spokesman for the Department for Culture, Media and Sport, said: “These ridiculous claims come from someone who has had no connection whatsoever with the 2012 project for almost 18 months.

“It is simply untrue to say that the real budget is £12 billion and that this figure has been suppressed.”

Tessa Jowell, the Olympics Minister, said: “This is an important milestone met and illustrates that the project remains firmly on track. The aquatics centre and stadium will be the showcase venues for 2012 and I am delighted that British companies will build them.”

How prices have risen since 2004

Total 2012 budget up from £3.3 billion to £9.3 billion including contingency

Olympic stadium up from £280 million to £496 million

Aquatic centre up from £75million to £303million (including £60 million bridge)

Velodrome park up from £28million to £80 million (including BMX circuit)

Security costs outside the park up from £600m to £838m

Construction inflation at 5% a year expected to add 50% to venue costs by 2012

Source: ODA