The head of the "cash for honours" investigation said it took so long because key people would not co-operate.

Metropolitan Police Assistant Commissioner John Yates' inquiry lasted 16 months.

Giving evidence to MPs investigating propriety in the honours system, Mr Yates outlined the immense pressure he was under as the lead detective in the controversial inquiry.

He indicated Tony Blair's aides had warned him the then Prime Minister was prepared to resign if he was questioned as a suspect in the case.

Mr Yates said he always had the full co-operation of the Cabinet Office, but there were some people who were not so helpful.

He said: "I don't think people deliberately misled us, but I do think in retrospect and with hindsight we were treated as a political problem, not a criminal problem."

Asked if Mr Blair or those around him had warned he could quit as a way of exerting pressure, Mr Yates said: "I would expect anybody looking after the interests of anyone at that level to set out a range of consequences to someone like me, in my position, who is managing the entire investigation, of the risk impact to the Metropolitan Police and beyond.

"If there was improper pressure that was put on the detective sergeant, for example, at the interview, that would be wholly improper.

"But to say to me 'Mr Yates, there are a range of consequences you might want to consider here' - quite proper, nothing in doubt about that. Did that pressure cause me to change any of my decisions? No it did not. It never would."

During a two-hour grilling by MPs, Mr Yates defended the police investigation, saying it was important his impartiality was not "compromised".

After a police investigation lasting more than a year, the Crown Prosecution Service announced in July that no charges would be brought in relation to the Labour Party.

It emerged last week that there would be no prosecutions relating to Tory lenders, either.

A spokeswoman for the committee has said it is not interested in re-running the police inquiry, but in finding out what lessons could be learnt in terms of possible changes to legislation.

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