Detectives investigating the "plebgate" row involving former chief whip Andrew Mitchel have arrested a 46-year-old female police officer.
The officer, who works in the Diplomatic Protection Group [DPG] at the Metropolitan Police Service, was arrested at her workplace in central London on suspicion of misconduct in a public office.
It is alleged the officer leaked information to the media which led to Mitchell resigning from his position as chief whip, over claims he called officers "plebs" outside Downing Street after they refused to let him through the main gates on his bicycle.
The arrested officer was present at Downing Street during the alleged incident involving Mitchell and the police officers.
This is the fourth arrest in the investigation, which is known as Operation Alice. A 46-year-old male officer arrested on 31 January has been bailed to return in March and suspended from duty, the Met Police said.
He was arrested on suspicion of misconduct in a public office and the unauthorised disclosure of information to the media. The Independent Police Complaints Commission (IPCC) haS been informed, Scotland Yard added.
Two other men - a 52-year-old DPG officer arrested on suspicion of misconduct in public office and a 23-year-old man arrested on suspicion of intentionally encouraging or assisting the commission of an indictable offence - were also arrested and later released on bail.
The investigation into the incident, which a total of 30 officers are now working on, was launched as new evidence cast further doubt on the official police version of events.
Mitchell, who always denied calling the officers "plebs" but admitted to saying "I thought you were here to f*****g help us", resigned nearly a month after the incident following media pressure.
CCTV footage appears to contradict the police's version of events, notably a comment which suggested "several members of public [were] present" at the time of the incident and were "visibly shocked". However, the footage shows there was only one member of the public at the scene, who did not appear to take too much interest in the exchange.
The CCTV footage also suggests the row went on for a shorter period of time than the witness statements claim.
All 800 members of the Metropolitan Police's diplomatic protection group have been asked to give statements as part of the investigation.