Great Britain's target of finishing fourth in the medals table at the 2012 Olympic Games in London is at risk because of a funding strategy that depends on £100 million of private money that “may not materialise”, the public spending watchdog announces today.

The National Audit Office (NAO) criticises the Government's decision to seek a sixth of the £600 million package promised for elite sport from commercial routes such as sponsorship. The Department for Culture, Media and Sport (DCMS) appointed Fast Track, the sports consultant, to raise the extra money but has not specified how it would do so in a tighter economic environment where competition for sponsors is fierce, not least from the London Olympic Organising Committee.

The NAO report concludes that the aim of raising the additional cash may not be achievable. “A key risk is that the £100 million, 17 per cent of direct funding for sports, which the department plans to raise from the private sector, may not materialise or may become available too late to influence Great Britain teams' chances of success at the London Games,” the report says. “UK Sport will need to take firm decisions on which sports to stop funding and when, if it is to minimise the impact of such a funding shortfall on medal performance in 2012.”

The findings highlight an outstanding issue about 2012 Olympic funding that is exercising senior ministers and alarming the IOC. British Olympic officials have “ultimate goals” of finishing fourth in the medals table in London and second at the Paralympics. To meet these aims, they secured an extra £300 million, with £200 million from the Exchequer and £100 million from the private sector.

One of the NAO's key recommendations is that the DCMS quickly assesses the appetite among potential commercial donors so UK Sport can distribute money to individual sports under its next funding round in April 2009 knowing the full extent of its resources.

“Two years after Gordon Brown trumpeted an extra £300 million for elite sport in his March 2006 Budget, it is deeply shaming that UK Sport still have absolutely no guarantee that one third of that money will be forthcoming,” Hugh Robertson, the Shadow Olympics Minister, said. “The Prime Minister has let down both UK Sport and the individual athletes badly.”

Another key finding of the NAO report is that Britain risks jeopardising its 2012 medals target by funding sports that have little chance of producing a podium finish. The recommendation undermines the decision by Olympic officials to compete in every discipline in 2012, regardless of Britain's pedigree in the sport. They maintain that Britain should fully participate in the Games, following the example of Australia in 2000 as host nations automatically qualify for every event.

However, the report concludes that full participation may dilute UK Sport's “no compromise” approach announced in February 2005 before it secured the extra funding. “There is a risk that the wider goals, in particular their aim to help develop Great Britain teams which can compete creditably in every Olympic and Paralympic sport at London 2012, may distract UK Sport's focus and funding from its primary goal of winning medals,” the report says.

This highlights the risks of earmarking £3 million for handball until 2009 - essentially for a team to learn the game in Denmark - rather than putting more resources into sports with a track record. The NAO says that the “host nation effect” would not be enough to double Britain's gold medal tally of nine in Athens in 2004, when the team finished tenth in the table. This would require a “step change” in the performance of elite athletes.

John Steele, the UK Sport chief executive, emphasised that the NAO had found “significant progress” in the funding of elite sport. “We recognise, too, the risks they highlight - in particular the fact that our ambitions are dependent on the full funding package being available,” he said.