For John Yates, it was an uncomfortable seven minutes.

After interviewing 136 people in an investigation that lasted 16 months and cost the taxpayer almost £1 milion, the Assistant Commissioner of Scotland Yard was left with the unenviable task of explaining why the political corruption inquiry had ended without a single conviction.

Sweating uncomfortably under the television lights, Mr Yates insisted that the decision to investigate had not been taken lightly.

“The police role in these matters is to gather evidence, analyse it and present a case to the CPS in order to prove or disprove the allegations,” he said. “It is a search for the truth.”

But Mr Yates, who is facing a parliamentary probe into his handling of the investigation, hinted that the blame for the failure lay elsewhere.

In a pointed aside, he said that his officers had for many months been “seeking the advice and views” of the Crown Prosecution Service (CPS) – the very body that appeared yesterday to suggest that a prosecution was out of reach.

The CPS revealed yesterday that it would not bring charges unless there had been an “unambiguous agreement” between two people that a gift would be made in exchange for an honour. “Such an agreement might be proved by direct evidence, or by inferences that can be drawn from the circumstances of the case. Such inferences must be so strong as to overwhelm any other, innocent, inferences that might be drawn from the same circumstances.

“There is no direct evidence of any such agreement between any two people subject of this investigation.” Legal experts suggested that this created an almost impossibly high hurdle to clear to secure prosecution.

Behind the scenes, this prompted fury from senior officers on the case.

They said they were surprised that the CPS had not advised the police earlier of apparent holes in the inquiry. One suggested that the bar for evidence had been lifted recently.

“If Carmen Dowd [head of the Special Crime Division at the CPS] was advising Yates’s team throughout the inquiry, when exactly did she tell them that there was not enough evidence? The collapse of this investigation is about bottle. John had lots of it, and the CPS did not,” a source said.

“There have been plenty of opportunities to inform police that they had insufficient evidence, but until recently, the CPS has been very encouraging. If they believe that their previous advice – that the investigation was going very well – was poor, they should be admitting their responsibility alongside Yates.”

Mr Yates was first given the job of overseeing the investigation by the Sir Ian Blair, Commissioner of the Metropolitan Police, last March. He was seen as a troubleshooter by colleagues after successfully handling the prosecution of Lord Archer of Weston-super-Mare, the investigation into the shooting of Jean Charles de Menezes, and more than 20 murder cases.

He has been in constant conversation with Ms Dowd and a team of lawyers from the CPS.

Mr Yates also headed the inquiry that jailed six corrupt officers known as the Groovy Gang in 2002.

In the same year he headed another high-profile inquiry into allegations of theft by Paul Burrell. This ended in embarrassment for the Yard when Diana, Princess of Wales’s former butler was cleared after the Queen remembered who had taken her posessions.

But it was this inquiry that friends say led to Mr Yates becoming so thorough in the cash for honours inquiry. “The Queen remembering something at such a late stage knocked him sidewards, and wasted a lot of police time,” one said. “He was determined not to miss a trick this time. If this one was to fail, it was not going to be because of a lack of effort.”

Tony Wright, the chairman of the Public Administration Committee, had private interviews with Scotland Yard as the investigation progressed. He said that it often seemed difficult to see where the police were heading.

“I thought at the outset that it would not lead to prosecution because I could not see that unless buying and selling of honours was going on in such an overt form – if you know the terrority, you would know that is not how it goes on – it is a far more covert world. It does not seem to me it would have led to a prosecution. And then it became clear as those avenues became more difficult [that] they were left trying to extricate something from it.

“On more than one occasion I remember asking him [Mr Yates] if he had found a smoking gun, and I don’t think he ever quite had. For a period they thought they were getting somewhere. Where they thought they were getting, I am not entirely sure.”

Secrets and loans

November 8, 2005 It is reported that millionaires who gave donations to the Labour and Conservative parties are to be given peerages in honours list

March 12, 2006 Chai Patel says he made £1.5 million loan to Labour

March 15 Jack Dromey, Labour Party treasurer, reveals he was not informed about secret loans

March 16 Angus MacNeil, the Scottish Nationalist MP, writes to police asking them to investigate

March 30 Scotland Yard says its inquiry is cross-party investigation

April 13 Headteacher Des Smith is first person arrested

July 8 The Times reveals, left, that Lord Levy told Sir Gulam Noon to hide the fact that he had lent £250,000 to the party after it had nominated him for a peerage

July 12 Labour’s chief fundraiser Lord Levy is arrested, below

September 20 Biotech mogul Sir Christopher Evans, who lent £1million to Labour, is arrested

September 28 Ruth Turner is questioned under caution

October 1 Tory donor and car importer Bob Edmiston is reported by The Times to have been questioned under caution

October 23 Michael Howard says he agreed to an interview by police

November 16 John Yates says inquiry has turned up “significant and valuable material”

December 14 Tony Blair is questioned, not under caution

December 18 The Times reveals investigation widened to include perversion of the course of justice

January 19, 2007 Miss Turner is arrested on suspicion of perverting the course of justice

January 24 John McTernan, political relations director at No 10, is reinterviewed by police

January 30 Lord Levy is arrested on suspicion of conspiracy to pervert the course of justice

February 1 No 10 discloses that Mr Blair was questioned for a second time on January 26

February 20 Ruth Turner answers police bail and faces questions

March 15 Mr Yates tells MPs it would be “unrealistic” to set a deadline for completing the inquiry

April 20 Detectives hand over their main evidence file

June 5 Scotland Yard reveals that the probe into the affair has cost more than £750,000

June 28 It emerges that former Mr Blair has been questioned for a third time by detectives.

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