The police chief who led the “cash for honours” inquiry accused Tony Blair and his allies yesterday of obstructing him and blamed them for extending his investigation by months.

Assistant Commissioner John Yates accused people at the top of Mr Blair’s administration of seeing him as a “political problem” and complained of the time that it took to establish key facts.

After giving evidence to MPs for two hours Mr Yates failed conspicuously to answer when asked whether, having spent 18 months in the “murky corners” of the British political Establishment, he believed that there was a “trade in honours”.

Mr Yates appeared to confirm that a warning was relayed to him at one point that Mr Blair would resign if he was interviewed under caution as part of the inquiry. In the event Mr Blair was questioned three times by police but on each occasion as a witness rather than as a potential suspect, although Mr Yates insisted that such warnings had not influenced him.

Asked by David Burrowes, a Tory MP, if people around Mr Blair gave warning that he would have to resign if interviewed as a suspect, he confirmed that the “consequences” were made clear to him. “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,” Mr Yates said.

Mr Yates, who was appearing before the Public Administration Select Committee, said he had hoped initially that his inquiry would take six months, but it ran for a year, followed by another four months until the CPS said in July that no charges would be brought. It took him ten months to establish how the list of working peers at the centre of the inquiry, four of whom gave secret loans to Labour before the 2005 general election, was drawn up.

“The Cabinet Office cooperated in full throughout,” Mr Yates told MPs pointedly. “Others did not. It is quite obvious to everybody who that was.”

Asked if he was referring to Downing Street, he replied that he had come to realise that the term Downing Street had “a number of meanings”.

At the start of the hearing, Mr Yates read a statement to the committee saying that he had received requests from lawyers representing all the witnesses and potential suspects expressing concern that he might reveal information gathered during his inquiry to which they could not respond.

During questioning Mr Yates defended vigorously the decision to set up the inquiry, only the second to be held under the 1925 Honours (Prevention of Abuses) Act, which bans the sale of honours, saying that there was a clear public interest at stake and he could not have known what evidence he would find unless he investigated.

At one point Tony Wright, the committee chairman, intervened to tell Mr Yates to “calm down” after he dismissed as “absolute nonsense” a question asking whether a police press officer working on the case would have leaked news of Ruth Turner’s arrest at her home in a dawn raid.

Earlier Mr Yates called for greater openness in the process used by party leaders to nominate supporters as peers. In investigating allegations of impropriety he had been hindered by the lack of any clear processes or records, he said. Mr Yates also appeared to call for leaders’ powers of patronage to be curtailed. “It leaves a lingering suspicion when you have the ability to nominate just like that.”

Mr Yates was robust in the defence of his investigation. “It would have been a disaster if I had done a less than thorough job. It would have been a disaster if my independence had been compromised.”

He said the nature of the offence of selling honours made it very difficult to prove. “They are bargains made in secret, both parties have an absolute vested interest in making sure those secrets don’t come out.”

Asked if he had felt under political pressure, he said: “Of course there was political pressure. It would have been inhuman of me not to have felt it.”

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