Ben Bradshaw, a former BBC journalist, said in a speech to the Royal Television Society Convention last night that he did not think the current model was sustainable and that he knew "no other area of public life where, as is the case with Trust, the same body is both regulator and cheerleader".
The BBC Trust was set up by the Labour government only three years ago after the statutory Charter Review, and Lyons said that it should be judged on its performance when the next review is due in 2013.
Lyons said in response to Bradshaw: "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, and if that means upsetting a minister along the way, it is unfortunate but so be it."
In the same speech, Bradshaw came out in support of some of the comments made by BSkyB chief executive James Murdoch in a speech at Edinburgh last month, where he attacked the BBC, saying: "The scope of its activities and ambitions is chilling."
Bradshaw said: "He was right to raise questions about the BBC's size, its remit and its impact on the rest of the British media industry," adding later that he thought the BBC had "probably reached the limits of reasonable expansion".
The full text of Bradshaw's speech can be read here.