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.
Mr Mitchell resigned as chief whip in October after weeks of controversy over an angry exchange after being told by police in Downing Street that he could not ride his bicycle through the gates.
He admits swearing at an officer but denies calling him a "pleb" or a "moron", insisting parts of a police log of the incident published in the media were "false".
"The IPCC received a referral from the MPS today and following an assessment of the available information, a decision was made to supervise the matter," the IPCC said yesterday.
"The investigation is considering the validity of the officer's claim, which is understood to have been made to a Member of Parliament in a private capacity."
It said the investigation was "linked to inquiries by the MPS (Metropolitan Police Service) into how internal MPS information was obtained by national newspapers following the incident".
Its involvement does not extend to that specific area of the case, however.
The watchdog can choose to supervise some elements of an investigation while leaving others - such as where it considers something to be an internal disciplinary matter - to a force.
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.
Mr Mitchell accepts that his parting shot to the on-duty officer was: "I thought you guys were supposed to f****** help us."
But he continues to contest other elements of what he is alleged to have said.
"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 yesterday.
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".
The arrested officer has been suspended from duty.
His treatment was questioned by Met Police Federation chairman John Tully who said an arrest appeared inappropriate and could be the subject of legal challenge.
"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 Telegraph.
"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."
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."