Updated: 21:51, 17 December 2012 - IPCC information
A police watchdog is investigating the "validity" of an arrested police officer's claim to an MP to have witnessed a foul-mouthed tirade by ex-cabinet minister Andrew Mitchell, it said.
The Independent Police Complaints Commission (IPCC) has decided to supervise part of a Scotland Yard probe into claims the officer "independently witnessed" the Downing Street incident.
It was called in after a Diplomatic Protection Squad member was arrested at the weekend - and suspended from duty - on suspicion of misconduct in public office.
Scotland Yard said it acted after receiving fresh evidence during a leak inquiry into how internal details of the so-called "plebgate" incident appeared in national newspapers.
The IPCC news came shortly after the officer's arrest was criticised by rank-and-file officers' representatives.
Met Police Federation chair John Tully questioned whether it was proportionate as the fallout continued from the foul-mouthed outburst which forced Mr Mitchell to quit the cabinet.
A Diplomatic Protection Squad officer was arrested on Saturday on suspicion of misconduct in public office. He has been suspended from duty and bailed to return in January.
As the case was referred to the Independent Police Complaints (IPCC) watchdog, Mr Mitchell renewed his claim that elements of a police log obtained by newspapers were "false".
The MP resigned in October after weeks of controversy over what he was reported to have said to police after being told he could not ride his bike through the main gates of Downing Street.
In his resignation letter, he admitted swearing at one of the officers on duty at the famous address - delivering as his parting shot: "I thought you guys were supposed to f****** help us."
But he firmly denied directing the words "pleb" or "moron" at him.
The story of the set-to first emerged in The Sun and transcripts of what was allegedly said, including those insults, appeared later in the Daily Telegraph.
Fresh information was obtained by the Metropolitan Police on Thursday about how the press came by the published information, leading to the arrest.
A Scotland Yard spokesman said there was "no evidence to suggest any of the officers involved in the incident were involved in the unauthorised release of information".
Mr Tully told the Daily Telegraph that lawyers may argue that the arrest was disproportionate under present guidelines and that the officer could have been interviewed under caution.
"The thing which disappointed me is around the proportionality of whether it was necessary to arrest the individual. After all, he is a serving police officer," he told the newspaper.
"Clearly it needs to be resolved, because there are things that we shouldn't talk about as police officers, and this may be one of those cases."
The Met's Directorate of Professional Standards which made the arrest had a "disproportionate view of what was proportionate"," he suggested.
Mr Mitchell had hailed the fact that the officer at whom he swore had accepted his apology for the foul language as a key defence as he sought to hold on to his post.
And he continues to maintain that he never used the word "pleb" - the word which became the focus of the furore which eventually forced him from office.
His refusal to accept the police account angered many officers, with Federation representatives accusing him of impugning the integrity of their colleagues who were on the scene.
Asked about the arrest, Mr Mitchell repeated his denial.
"I'd just like to reiterate once again, that it's the contents of the alleged police log which are false... they are false and I want to make that very clear," he told ITV News.
The prime minister's official spokesman said: "At the time we said there was a genuine difference of opinion.
"Clearly this is something the police are dealing with and I don't think I want to get drawn into further comment."
Asked if the prime minister was concerned about the arrest of the whistle-blower, the spokesman added: "That arrest is a matter for the Metropolitan Police."
Met Commissioner Bernard Hogan-Howe said: "The investigation has been referred to IPCC to oversee this investigation and it wouldn't be possible to go into too much detail to explain all the background into this particular case while the case is ongoing.
"It's vitally important that we allow the investigators to get on with the investigation, to discover what they are going to find out and then a decision will be made about whether there should be a prosecution, or alternatively whether any misconduct case should come, or none at all."