Sir Michael Lyons was appointed chairman of the BBC Trust last year
BBC Trust chairman Sir Michael Lyons has spoken out against calls to use the licence fee to help fund Channel 4.
He told the Royal Society of Arts that "well-meaning" intervention could risk turning Channel 4 into "BBC Five".
Any debate about the future of public service broadcasting, he said, must be in the interest of audiences, and not broadcasters - including the BBC.
Using some of the licence fee to fund unprofitable Channel 4 shows was recently suggested by Ofcom.
The plan, the watchdog said, would help the BBC's rivals pay for public service programming, such as news and children's shows.
But Sir Michael said there was no "excess" licence fee and if such a thing did exist, it should be up to viewers to decide what to do with it.
He argued that giving Channel 4 public money could "weaken rather than cure the patient" and that it would lead to a change in the character of the broadcaster.
"Has Channel 4's audience been properly consulted about the risks such an arrangement might entail?" he said.
"Put bluntly, the question is this. Who gains if the effect of well-meaning regulatory intervention is to turn Channel 4 into BBC Five?
"The debate about the future of public service broadcasting in the UK must be about much broader issues than the future of Channel 4."
Sir Michael also said the BBC could "improve on its record of reaching ethnic minority audiences".
He cited the recent adaptation of The No 1 Ladies' Detective Agency, adding that it drew a much larger black audience than average for BBC One.
"So it is possible to bring diverse audiences together to share a common BBC experience, if the BBC is prepared to take the necessary risks, in this case showcasing at primetime on the flagship channel an entirely black cast with Botswanan accents in an African setting."
The BBC had "the scale and scope" to be able to offer targeted channels such as the Asian Network, or BBC 1Xtra, the digital radio station for young black urban audiences.
"But the need to reach everyone should not be an excuse to go on creating more services. The BBC must also find ways to bring those audiences into mainstream output," he added.