Moment of truth as Yates of the Yard is questioned over cash-for-honours

Assistant Commissioner John Yates will be challenged by MPs over allegations that Downing Street "obstructed" the inquiry.

At the height of the investigation, claims emerged that Mr Blair had threatened to resign as Prime Minister if he was quizzed under caution as a suspect rather than simply as a witness.

In the end, Mr Blair's status remained that of a witness throughout the 16-month investigation into claims that Labour "sold" peerages to wealthy businessmen.

The interrogation of Mr Yates and senior CPS lawyer Carmen Dowd by the Commons public administration committee tomorrow will also cover why the inquiry dragged on for so long, costing taxpayers around £1million. It ended when the Crown Prosecution Service decided not to charge anybody.

Detectives involved in the inquiry were privately furious at the decision.

Mr Yates, a Scotland Yard Assistant Commissioner, is set to defend the quality of his evidence to the MPs.

He has already promised to make it all available to them if they want to see it.

Scotland Yard has never commented on the claims that Downing Street officials "interfered" in the inquiry.

Paul Rowen, a Liberal Democrat member of the committee, said: "I want to know whether Assistant Commissioner Yates and his team got all the co-operation they were entitled to during their investigation.

"Was there obstruction from Downing Street or Mr Blair and why at the end of the day, when it was clear that Mr Blair had driven a coach and horses through the honours regulations, were they not able to do anything about it? The public has a right to know.

"Tony Blair has made a mockery of public life. We have heard allegations that he even threatened to resign if he was forced to answer questions under caution.

"These are the things we need to ask and we are going to be asking - what pressures were put on, how cooperative was Downing Street and why did it take so long to get nowhere?"

Mr Yates has refused to comment, but colleagues are speculating about whether his evidence will contain bombshell revelations about lack of co-operation from Downing Street.

"John is keeping his own counsel but this week's meeting promises to be very interesting," said one colleague.

"John is a guy of real integrity and he may now feel he can be open and straightforward about some of the things he and his team found out.

"I don't think he will throw accusations about loosely, but he might point out that there were unnecessary delays in obtaining some evidence."

Mr Yates will also be asked about why his officers carried out a dawn raid on the home of Ruth Turner, Tony Blair's former "gatekeeper".

Miss Turner was arrested along with Labour's chief fundraiser Lord Levy and millionaire donor Sir Christopher Evans. But none was charged.

During the hearing Mr Yates is expected to assert that Sir Christopher's diaries contained evidence seen as vitally important to his investigation.

They are thought to mention conversations between the biotechnology tycoon, who loaned Labour £1million, and Lord Levy, about the possibility of an honour.