A police officer has been arrested on suspicion of misconduct in public office over the leak of information to national newspapers about Tory former chief whip Andrew Mitchell's tirade at officers on Downing Street.
A constable in the Diplomatic Protection Group was held on Saturday evening and bailed on Sunday to return next month, Scotland Yard said.
Mr Mitchell 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.
On resigning, he insisted in a letter to Prime Minister David Cameron that he had not referred to an officer on the gate as either a "pleb" or a "moron" but acknowledged delivering the parting line: "I thought you guys were supposed to f****** help us."
The story first emerged in The Sun and transcripts of what was allegedly said appeared later in the Daily Telegraph.
Scotland Yard said that on Thursday the Metropolitan Police Service (MPS) received fresh information regarding the alleged unauthorised disclosure of information. As the result of this the Directorate of Professional Standards arrested a police constable in the Diplomatic Protection Group, on suspicion of misconduct in a public office, at 8.15pm on Saturday. He was bailed on Sunday morning to return in January. He has been suspended from duty. The matter will be formally referred to the Independent Police Complaints Commission on Monday.
The Yard's statement said the arrest was linked to previous inquiries by the MPS as to how internal information was obtained by national newspapers following an incident at Downing Street in September. "These inquiries found no evidence to suggest any of the officers involved in the incident were involved in the unauthorised release of information. The officer arrested was not on duty at the time of the incident in Downing Street."
When he resigned Mr Mitchell admitted the row over the incident made his position untenable. He said it was not fair to put his colleagues and family through such "damaging" stories any longer. He had clung desperately to his position amid a mounting clamour lasting a month for him to go.
Mr Mitchell 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."
At a regular Westminster briefing the Prime Minister's 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."