Auditors from the public spending watchdog are to be given greater freedom to inspect the BBC's books, under the terms of a deal to be announced on Wednesday.
In response to calls for more openness about how licence fee-payers' money is spent, it is understood that the BBC Trust agreed to accept changes which will allow the National Audit Office freedom to choose which areas of the corporation's operations it wants to investigate.
Until now, the NAO has only been able to scrutinise areas chosen by the Trust as part of the value-for-money studies of BBC activities it carries out as the corporation's governing body, and has got involved in the studies only at the Trust's invitation.
The terms of the deal will be announced by Liberal Democrat culture spokesman Don Foster at the party's conference in Liverpool.
Although the NAO will have the power to decide which of the BBC's operations it wants to scrutinise, restrictions are expected to remain on the publication of information about sensitive subjects - such as commercial deals with independent companies and the amount of money it pays to on-screen stars.
While the NAO reports directly to Parliament on its investigations into other public bodies, its findings on the BBC go to the Trust, which then presents its own report to MPs through the Commons Public Accounts Committee.
This arrangement, designed to preserve the BBC's independence from political pressure, is understood to be maintained under the new deal.
Discussions about opening the BBC up to greater scrutiny have been going on between the Trust and the Department for Culture, Media and Sport over the summer.
Earlier this month, Trust chairman Sir Michael Lyons indicated that he would be ready to countenance an expansion of the NAO's scrutiny role.
Giving evidence to the Commons Culture, Media and Sport Committee, Sir Michael said: "They have a strong voice in which areas to conduct their studies in, but at the moment those are subject to agreement by the Trust. It is not impossible to imagine them having greater freedom to name the areas they want to go in and I don't think the Trust would be unwilling to consider that as the way forward. There just need to be a few ground rules then about how those are conducted."