A female police officer tasked with protecting Government officials was today arrested over alleged leaks to the press about the so-called "plebgate affair".

The 46-year-old, who works in the Diplomatic Protection Group at the Metropolitan Police Service, was arrested at her workplace in central London on suspicion of misconduct in a public office.

She was arrested over alleged leaks to the media linked to the row which led former Tory chief whip Andrew Mitchell to quit his Cabinet post following claims he called officers "plebs".

The officer arrested was present at Downing Street during the incident in September, Scotland Yard confirmed. This was the fourth arrest in the investigation, which is known as Operation Alice.

A 46-year-old male officer arrested yesterday has been bailed to return on a date in March and has been suspended from duty, the Met Police said.

Three weeks ago Metropolitan Police commissioner Sir Bernard Hogan-Howe said that a file of evidence on the case could be handed to prosecutors by the end of January.

But Scotland Yard yesterday said that because of "ongoing developments" no interim report had yet been submitted to the Crown Prosecution Service.

Investigators have taken statements from all 800 officers in the Diplomatic Protection Group (DPG) as part of the inquiry.

Two other men have been arrested so far, both of whom have been released on bail until February.

A 52-year-old Pc who is also from the DPG and currently suspended was held on suspicion of misconduct in public office, while a 23-year-old was arrested on suspicion of intentionally encouraging or assisting the commission of an indictable offence.

Four other constables in the unit, which is responsible for protecting government officials and diplomats, have been placed on restricted duty over misconduct claims.

The plebgate row began after Mr Mitchell was accused of a heated rant against officers as he left Downing Street.

Pressure on him intensified after The Daily Telegraph published a police log of the incident, which claimed he called officers "plebs" and swore at them repeatedly.

He insisted he did not use the words attributed to him, and later said he was the victim of a deliberate attempt to "toxify" the Tories and ruin his career.

An email from a civilian witness backing up the police account of events was also called into question.

By January 16, Operation Alice, which has around 30 detectives working on it, had cost £82,500 since it was set up in December.

The figure, which related to staffing costs, was disclosed in a letter from deputy assistant commissioner Patricia Gallan to Keith Vaz MP, chairman of the Home Affairs Select Committee.

Sir Bernard last month told the committee he would "pursue the evidence".

He told MPs that part of the investigation was to identify the source of the leaked police log, which recounted the altercation with Mr Mitchell.

Sir Bernard criticised public statements made by Police Federation representatives in the wake of the initial allegations against Mr Mitchell.

Local Federation branches organised protests by members wearing "PC Pleb" T-shirts and demanded Mr Mitchell's sacking.