Bernard Hogan-Howe, the Metropolitan Police Commissioner, was under mounting pressure tonight after being accused of pre-judging the investigation by his force into the Andrew Mitchell affair.
The Commissioner came under fire as Mitchell allies and a former Director of Public Prosecutions questioned his decision to publicly support the two officers involved in an altercation with the former Government Chief Whip in Downing Street after fresh evidence doubting their claims emerged last week.
Mr Hogan-Howe, who interrupted his Christmas break to get a progress report on the investigation, responded to the criticism by pledging to seek out the truth.
He said in a statement: "The allegations in relation to this case are extremely serious. For the avoidance of doubt, I am determined there will be a ruthless search for the truth – no matter where the truth takes us."
He said the police force's determination to do that was proved by his decision to devote 30 officers to the task, and the arrest of a member of the diplomatic protection squad and a civilian.
Calling for the investigation to be allowed "time and space", he said: "I believe these actions are vital in maintaining public confidence in the police."
He pointed to his choice of Deputy Assistant Commissioner Patricia Gallan, head of Professional Standards, to head the investigation, adding that it was being supervised by the Independent Police Complaints Commission (IPCC).
Mr Hogan-Howe said that during the last 24 hours, he had "taken the opportunity to satisfy myself about the welfare of the officers involved" in the Mitchell incident.
Yesterday, Lord (Ken) Macdonald, the former Director of Public Prosecutions, said the Commissioner was "extremely foolish" to say he believed the two officers' claim that Mr Mitchell called them "plebs" as the investigation was announced.
Writing in the Mail on Sunday, he said: "What exactly does Bernard Hogan-Howe consider is the purpose of having an inquiry into the men he has already exonerated on television?"
Last week, the Commissioner said he had seen no evidence that "really affected the original account of the officers at the scene". These remarks also raised eyebrows among Mitchell supporters, with one former minister describing them as "astonishing."
Mr Hogan-Howe will face questions about his comments when he appears before the Commons Home Affairs Committee next month. Downing Street sources insisted the Commissioner retained the Government's full confidence.
Yesterday, Mr Mitchell gave his first full account of the controversy which cost him his job, claiming he was the victim of a conspiracy by "police elements".
He is planning to sue individuals and media organisations he accuses of blackening his name, and to donate any damages he wins to a hospice in his Sutton Coldfield constituency. His lawyers are studying 1,000 articles and tweets.
Some Conservative MPs, while expressing sympathy for Mr Mitchell, believe he has gone "over the top" in appearing to criticise David Cameron's handling of the crisis, saying that will not enhance his prospects of a speedy return to a Cabinet post. "The Prime Minister did not tell him to go immediately; he held on to him for weeks," one MP said. "A lot of things are now being said with the benefit of hindsight."
David Hanson, the shadow Policing Minister, expressed concern that Number 10 and the Cabinet Office had treated the affair "simply as a media handling issue". He said: "The IPCC should be able to follow the evidence wherever it takes them. For instance, why was this email from someone purporting to be an eyewitness not checked against the CCTV or given to the police months ago so they could investigate?
"In addition, given the current pressures on the IPCC, they will need to set out clearly their involvement and capacity to pursue this investigation fully to ensure it is independent and wide ranging enough."