The NAO also warned the final cost of the Games was "inherently uncertain", and the remaining contingency needed to be carefully managed to stay within the overall £9.3 billion budget, according to his latest report, the fifth it has published on the preparations for 2012.

"There can be no guarantee that the remaining contingency will be sufficient to cover further unknown risks to the Games," the NAO said in its latest 2012 Olympic progress report.

"The Government Olympic Executive should have plans for how it will meet any requirement for extra funding which cannot be met from within the remaining contingency."

The Olympic Delivery Authority (ODA), responsible for getting the Games ready, is currently on track to meet its budget and construction targets, with all 24 main infrastructure and venue projects due to be handed over to London 2012 for testing on or ahead of schedule.

Five have already been finished.

But construction on the Zaha Hadid-designed aquatics centre has slipped by two months to June 2011 because of complications with its wave-shaped roof.

Two of the 11 residential blocks in the athletes' village have taken 15 weeks longer than planned.

Question marks still surround funding for some projects, including security around transport hubs, while the NAO continued to express concern about London 2012 finances.

The overall cost to the public purse of security during the Games has increased from £600 million to £757 million.

This is mainly because the Government has now agreed to provide £282 million from the £9.3 billion Public Sector Funding Package for work to secure the perimeter of the Olympic Park and venues during the Games, offset by expenditure on wider security and policing which is now forecast to be £125 million less than it was in March 2007, the report said.

In its previous report the NAO had highlighted the need to finalise the costs and funding responsibilities for venue security.

The NAO also called for the non-sporting events programme around the Games to be finalised so their impact could be assessed on transport planning and security resources.

Legacy ambitions, a key plank of London's successful bid, may suffer because of the "natural priority" to deliver a successful Games within an immovable deadline.

For example, a pledge on improving the employment prospects and skills of women and disabled people had fallen short, the report said.

The Olympic Park Legacy Company, responsible for the venues after the Games, not only lacked an agreed funding package, but also did not have a clear plan on how the stadiums would be maintained if it failed to find anchor tenants.

"Good progress is being made in the preparations for the 2012 London Games which will begin in 17 months," said Amyas Morse, head of the NAO.

"All construction and infrastructure projects are forecast to be completed on time, albeit in two cases with little room to spare before the deadline for handover to LOCOG, the Games organiser, and operational planning has improved.

"However, the final cost of the Games to the taxpayer is inherently uncertain and as the Games near there will be less flexibility to make savings in response to any unforeseen financial pressures."

To read the full report click here.


Related stories
February 2010: National Audit report increases pressure to move West Ham into stadium
May 2008: London 2012 told to get a grip on costs