Armed police from the Diplomatic Protection Group are being investigated over suspicions they sent one another extreme pornographic images.

Three officers, whose duties included guarding Downing Street, have been arrested on suspicion of “being involved in the possession and distribution of obscene images” using mobile phones.

One of the arrested officers has been suspended from duty and the other two have been placed on restricted duties. A fourth officer has also been questioned as part of the investigation and is on restricted duties.

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The conclusions of the investigating team from the Directorate of Professional Standards has now been passed to the Crown Prosecution Service which is considering if the evidence is strong enough for charges to be pressed.

Scotland Yard issued a statement in which it described the pornographic images as extreme but said none of them involved children. The arrests were made in December but have only now been made public.

The images are said to have been discovered during Operation Alice, the inquiry prompted by the altercation that led to the then Government Chief Whip Andrew Mitchell being accused – and vociferously denying - of calling police at the gates of Downing Street “plebs”.

More than 700 police officers are in the Diplomatic Protection Group which has responsibility for the security of Downing Street, the Palace of Westminster, embassies and other sensitive sites.

Investigators probing the obscene images will, as part of the inquiry, check mobile phone records to establish if the pictures were sent while the protection officers were on duty.

Scotland Yard confirmed in its statement: “On 19 December 2013 officers from the DPS arrested 3 police constables from the Diplomatic Protection Group (SO6) on suspicion of being involved in the possession and distribution of obscene images via mobile phones.

“The images identified by the investigation are of an extreme sexual nature, but do not involve children.

“The arrested officers were taken to a central London police station for questioning. Search warrants have been executed at the officers’ home addresses. One officer has been suspended from duty. Two others have been placed on restricted duties.

“On 8 January 2014 a fourth officer was interviewed in connection with the investigation and placed on restricted duties.

“A file has now been passed to the CPS for consideration. The DPS misconduct investigation is on hold pending the CPS decision.”

The Metropolitan Police referred the matter to the Independent Police Complaints Commission (IPCC) which decided it should be investigated by the DPS.