Ben Bradshaw said it was right to debate the BBC's size
Culture secretary Ben Bradshaw has called for changes to the way the BBC is governed and said there may be "a case" for the licence fee to be cut.
Mr Bradshaw said the BBC Trust, which sets the corporation's strategy and upholds standards, could not be both "regulator and cheerleader" for ever.
The BBC may have reached the limits of its "reasonable expansion", he added.
The BBC Trust said it would continue to defend the corporation's independence and the licence fee payers' interests.
Mr Bradshaw, a former BBC journalist, has already clashed with the BBC since taking on the role in June.
He supports plans to use part of the TV licence fee to fund regional ITV news, a move known as "top-slicing", which BBC management is opposed to.
Mr Bradshaw told TV executives in Cambridge that the BBC Trust, set up three years ago with the backing of ministers, had performed "better" than the old board of governors.
But he added: "I am concerned about the regulatory structure of the BBC. I don't think that it is a sustainable model in the long term.
"I know of no other area of public life where - as is the case with the Trust - the body is both regulator and cheerleader."
While there may be a "case" for a smaller licence fee, Mr Bradshaw said now was not the time for such a discussion.
He said it would be completely wrong to revisit the current settlement, agreed in 2006 and lasting until 2012.
"One of the unbroken conventions adhered to by successive governments, to avoid the suggestion of political interference in or pressure on the BBC, has been to respect the multi-annual settlement system," he said.
"I resolutely believe that to be right."
But he indicated that the renewal of the BBC's Charter, which is due in 2016, must serve as an opportunity for people to have their say on the future role of the BBC and of public service broadcasting in general.
He called for the process to be a "proper national conversation, certainly not a stitch-up behind closed doors between BBC management and politicians".
"Only that way will whatever is agreed have the legitimacy to withstand the onslaught from the BBC's enemies and critics and stand the test of time."
Sir Michael Lyons, chairman of the BBC Trust, said the BBC would be "judged on its performance" at the time of Charter Review.
He added: "In the meantime, we have been set up to be, as the then secretary of state put it in 2006, "the voice, eyes and ears of licence fee payers".
"That means reshaping the BBC, defending its strength and independence and also protecting the investment licence fee payers have made. If that means upsetting a minister along the way, it is unfortunate but so be it."
The Conservatives accused Labour of a U-turn on the role of the BBC Trust, saying people would ask questions about whether there was "any consistency in media policy".
Mr Bradshaw's views over "top-slicing" has put him at odds with BBC management.
The government wants to use a small part of the money after 2012 to maintain regional news on ITV1, which can no longer afford to provide it.
The BBC says that would break the historic funding link between it and its audiences, reducing accountability.
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