The London Olympics will not be able to fulfil its promise of helping Britain's neediest young people, according to a report.
The legacy pledge will come to be seen as a "highly effective sales pitch that was never fully realised", the Centre for Social Justice think tank said.
Promising a sporting legacy across the country lay at the heart of the London 2012 bid.
But the report said: "The legacy promise will come in time to be viewed as a highly effective sales pitch that was never fully realised.
"The scale of the challenge that the Olympic organisers have set themselves is too high for the relatively limited amounts of funding and the programmes that have been promised, to deliver successfully."
In November, a £135 million Lottery-funded drive was announced to encourage mass participation in sport on the back of the London Games.
Olympics Minister Hugh Robertson had said tackling the poor state of sports facilities across the country is a top priority.
A nationwide campaign to offer teenagers and young adults six weeks of coaching in the sport of their choice was also announced.
But the Centre for Social Justice said upgrading facilities will not in itself increase participation, adding that taster coaching courses had failed to engage disadvantaged groups.
The report said: "Limited available funding and the tendency to direct what there is into capital spending and short-term programming mean that it is difficult to see how the money allocated to this can be expected to produce greater benefits for disadvantaged young people.
"The participation target was intrinsically flawed from the outset, not just because it was more convincing as a sales pitch than a policy objective, but also because engaging any number of additional people in some unspecified sporting activity is not the same thing as a serious, targeted work aimed at transforming the lives of Britain's neediest people."
The report also said previous Olympics, such as the Sydney Games in Australia failed to produce an increase in participation, and pointed out that popular school sports such as cricket, rounders and netball will not feature in London 2012. It added that there was no evidence of a link between national sporting success and increased levels of sporting activity.
The full report, More Than A Game: Harnessing the power of sport to transform the lives of disadvantaged young people, will be formally launched on Tuesday.