Honours probe man is 'gagged'

John Yates faces questions from MPs over why the inquiry, thought to have cost taxpayers about £1 million, dragged on for 18 months, when no charges were brought.

But a ruling that he cannot disclose key evidence if it would infringe the rights of people cleared of criminal conduct means the public may never learn why detectives seemed so convinced there was a case to answer.

Mr Yates is due to be interviewed by the Public Administration Select Committee and will be asked to justify the way the case was handled by the Metropolitan Police.

MPs will ask whether police fell victim to a political stunt, arguing that a conviction was virtually impossible because of the high standard of evidence required in such cases.

But others on the committee want to know what led police to believe peerages may have been dangled as inducements to Labour donors.

A linked investigation was carried out into whether senior Downing Street officials attempted a cover-up.

At the mystery's heart is believed to be a diary written by bio-tycoon and Labour donor Sir Christopher Evans, which was rumoured to record a conversation about a "big P" - thought to mean a peerage - with Tony Blair's fundraiser Lord Levy.

At the last minute, the document was ruled inadmissible as evidence, ripping a hole in the police case. Both men were cleared of any wrongdoing.

The committee is said to have rebuffed a request by Mr Yates to give evidence in private, putting extra restraint on how much he can say.