Question marks still hang over the size of the final cost to the taxpayer of the London 2012 Olympics, the Government's spending watchdog has said.

There is no guarantee the Olympic Games project will come in on budget

The National Audit Office (NAO), in its latest report on preparations for the Games, notes the final cost is "inherently uncertain".

The timings for handover of the £269m Aquatics Centre in July and the Athletes Village in January 2012 are now "becoming tight," the NAO said.

However, five of the Olympic Delivery Authority's 24 main building projects are finished and confidence is high all the others will be ready on time.

The overall cost to the public purse of security during the Games has increased from £600m to £757m, the NAO reports.

This is mainly because the Government has agreed to make available another £282m to help secure the perimeter of the Olympic Park and other venues.

The NAO, which had called for final figures on the estimated £9.3bn overall cost of the Games, notes some progress has been made.

Of the original £2.747bn of taxpayers money set aside to help pay for the Games £974m remains, and with almost 80% of the construction programme complete the chances of overspending have been reduced.

But the NAO warns there is no guarantee the project will come in on budget and urges ministers to work out where any extra cash needed might come from.

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

"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 and operational planning has improved.

There are are concerns about how the Olympic legacy will be implemented

"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."

The NAO has also raised concerns about legacy, noting there are no arrangements in place to monitor its delivery after the Games.

That concern is shared by Sir Robin Wales, mayor of the east London host borough of Newham.

He said the Games will be "nothing but a vanity parade" if there is not a meaningful legacy.