The government must reveal how much money is to be diverted from heritage projects to pay for the London Olympics in 2012, MPs have said.
MPs asked how much could be recouped from selling Olympic sites
Official estimates suggest the Heritage Lottery Fund's annual income will fall by £57m to £180m by 2009/10.
But the Commons public accounts committee said there were "major areas of uncertainty" and called for a "frank assessment" of the situation.
The government said it would consider the MPs' recommendations.
Ministers have been criticised by heritage, sports and community groups for "raiding" the fund to help pay for the 2012 Olympic and Paralympic Games.
The National Lottery is committed to raising £750m towards the cost, but the government insists no existing good cause commitments should be affected.
However, the committee warned that the fund's income could drop by a further £95m if special Olympic lottery games - with "ring-fenced" money for the Games - divert sales away from other tickets and scratch cards.
In its report it said there was also "no guarantee" the fund would recoup the money lost when land developed for the Olympics is sold after 2012.
The committee's chairman, Conservative MP Edward Leigh, said: "The fund will have to work hard to make ends meet.
"With less money to go round, the fund should look harder at whether applicants have exhausted other sources of funding. And it must have a sharper sense of what its funding is achieving.
"The fund must also ensure that it distributes money as fairly as possible.
"It needs to stimulate good quality applications from hitherto under-represented groups so that they gain access to lottery funding; and it must work to remove bureaucratic hurdles faced by applicants."
Liberal Democrat arts and culture spokesman Dan Rogerson said the government was "putting the nation's heritage at risk".
He added: "Lottery tickets must be clearly labelled so that people can make genuinely informed choices between further funding for the Olympics and supporting community heritage projects."
A Department for Culture, Media and Sport spokeswoman said: "We note the committee's findings and we will consider them."