Mr Ainsworth's letter to John Birt suggests that the trade and industry secretary has been singled out for special treatment.

It reads: "Concern arises as to why it was thought uniquely necessary to apply a code of silence to an individual cabinet minister."

Peter Mandelson: Instructions not to mention his private life

Mr Ainsworth said earlier that the question of Mr Mandelson's private life was a matter of utter indifference to him and in the absence of the memorandum, would have remained so.

Instructions to BBC staff not to refer to Mr Mandelson's private life followed remarks made about him by columnist and former Tory MP, Matthew Parris, on Newsnight.

A memo was issued to all BBC programme makers, stating that details of Mr Mandelson's personal life were not to be broadcast.

Mr Ainsworth adds: "I would be grateful if you could confirm that at no time were representations made wither by, of on behalf of the government, to the BBC in relation to issuing these instructions."

He also asks whether there is a blanket ban within the BBC "not to permit references to the private lives of politicians generally, where there is no public interest involved, or whether that policy applies to Mr Mandelson alone".

Peter Ainsworth: Blanket ban on mentioning private lives?

The Tories say two members of the shadow cabinet have also been requested to obey the gagging order in interviews with the BBC.

Mr Ainsworth adds: "This is plain ludicrous. I would suggest that in the interests of all concerned, you withdraw the instruction ... they have proved to be self-defeating and unworkable and have brought no credit to the BBC."

A BBC spokeswoman said: "We cannot offer any comment. We are not aware that we have received any formal complaint yet from Mr Ainsworth.

"In the event of us having any formal complaint we would respond to it appropriately."

The spokeswoman said the memo was sent out to remind programme makers of the BBC's own producer guidelines, which stipulate that a person's sexuality is not a matter for comment unless it is in public interest.