Three of the armed police officers assigned to guard Downing Street who were investigated over allegations that they used their mobile phones to exchange extreme pornography will not face criminal charges.

Three officers from the diplomatic protection group (SO6), a unit already in the spotlight recently as a result of the Plebgate affair that led to the resignation of cabinet minister Andrew Mitchell, were arrested on 19 December, Scotland Yard confirmed.

A fourth officer who was not arrested was interviewed on 8 January in connection with the investigation and placed on restricted duties.

The Metropolitan police said that the images identified by the investigation were of an extreme sexual nature but did not involve children and that a file had been passed to the Crown Prosecution Service (CPS) for consideration. Scotland Yard said on Saturday three officers will not face charges but the CPS had yet to make a decision on the fourth officer.

Searches were carried out at the home addresses of the arrested officers, who were questioned at a central London police station. One of them has been suspended from duty while two others have been placed on restricted duties.

The force said that it had voluntarily referred the matter to the Independent Police Complaints Commission, which instructed that it should be investigated the Met's own directorate of professional standards (DPS), the unit responsible for investigating complaints against the professional conduct of officers.

It added in a statement: "The DPS misconduct investigation is on hold pending the CPS decision. The misconduct investigation will examine whether the conduct allegedly took place whilst the officers were on or off duty conduct."

The Diplomatic Protection Group is responsible for, among other duties, guarding embassies and missions, protecting visiting heads of state as well as providing specialist search officers for counterterrorism searches at major events and crime scenes.

Seven Metropolitan police officers from the group are facing disciplinary action over the Plebgate affair, which erupted when Mitchell confronted an officer after he was refused permission to cycle through the main gate of Downing Street. The MP admitted swearing but has strenuously denied Rowland's claim that he used the word "pleb".

Of the seven, four will be subject to gross misconduct hearings in March over claims that they leaked information to the media.

Another former member of the diplomatic protection group was jailed on Thursday after he admitted lying about witnessing the Plebgate incident.

Constable Keith Wallis, who was sentenced to 12 months in prison when he appeared at the Old Bailey in central London, is the only police officer to have admitted wrongdoing over the saga.

Wallis was off duty on the day of the incident, the court heard, but had picked up on "office gossip and rumour" about what Mitchell had said when he returned to work the next day.

Scotland Yard has said previously that Wallis, who has offered to resign from the force, would face a misconduct hearing following the conclusion of the legal proceedings.

The Metropolitan police commissioner apologised in person to Mitchell on the eve of the sentencing of Wallis.

Sir Bernard Hogan-Howe held a 50-minute meeting with the former chief whip in his Commons office, where they agreed on the importance of "drawing a line" under the matter.