A police officer who claimed to have witnessed Andrew Mitchell's Downing Street rant is being investigated by the police watchdog.
The Independent Police Complaints Commission (IPCC) is looking into the "validity" of his account - which helped lead to Mr Mitchell's resignation as Tory Chief Whip in October.
It has decided to supervise part of a Scotland Yard probe into the claims that the officer "independently witnessed" the argument.
The watchdog 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 have made clear that the officer was not on duty at the time of the clash, which erupted when Mr Mitchell was told he could not cycle through the main Downing Street gates.
Police acted after receiving fresh evidence during an inquiry into how internal details of the "plebgate" row were leaked to the press.
The story first emerged in The Sun and transcripts of what was allegedly said appeared later in the Daily Telegraph.
Mr Mitchell admits swearing at an officer but he denies calling him a "pleb" or "moron" and has repeatedly insisted parts of the police log of the clash are inaccurate.
On Monday, he told ITV News: "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."
A spokesman for the police watchdog said: "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 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".
However, it will not look at the leak itself - leaving this as an internal disciplinary matter for Scotland Yard.
The latest development raises questions about whether there was ever independent corroboration of the accusation that the senior Tory MP called police "plebs".
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.
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."