A police officer who claimed to have witnessed Sutton Coldfield MP Andrew Mitchell's rant at Downing Street is being investigated by the police watchdog.
The Independent Police Complaints Commission (IPCC) has decided to supervise a Scotland Yard probe into claims the officer "independently witnessed" the Downing Street incident.
A Diplomatic Protection Squad member was arrested and suspended from duty on suspicion of misconduct in public office.
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".
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.
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".
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: