LONDON (AFP) - Three police officers from Britain's Diplomatic Protection Group (DPG) have been arrested on suspicion of exchanging hardcore pornography on their mobile phones, Scotland Yard said on Saturday.

The DPG controls access to the prime minister's Downing Street residence, and Scotland Yard, the police headquarters in London. It also guards embassies in the capital.

"The images identified by the investigation are of an extreme sexual nature, but do not involve children," said a Scotland Yard spokesman.

The three men were arrested on Dec 19. One of the trio has been suspended while the other two have been placed on restricted duties. Their homes have also been searched.

A fourth officer, who was interviewed on Jan 8 in connection with the investigation, has also been placed on restricted duties.

The DPG also protects government ministers, former premiers, some former government ministers, and visiting heads of state and government.

The Scotland Yard spokesman said a file had been passed to the state prosecutors, with a police misconduct probe on hold until after prosecutors decide whether the three should be charged.

"On Dec 19, 2013 officers from the Directorate of Professional Standards (DPS) arrested three police constables from the Diplomatic Protection Group on suspicion of being involved in the possession and distribution of obscene images via mobile phones," he said.

"This is contrary to both the Obscene Publications Act 1959 and also the Criminal Justice and Immigration Act 2008 (section 63-67).

"A file has now been passed to the Crown Prosecution Service (CPS) for consideration.

"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."