LONDON (AFP) - Three police officers from the elite squad that protects Downing Street who were arrested over allegations they exchanged hardcore pornography on their mobile phones will not face criminal charges, Scotland Yard said on Saturday, Feb 8, 2014.

The three men, who were arrested on Dec 19, were questioned at a police station over images "of an extreme sexual nature" and had their homes searched.

They were from the Diplomatic Protection Group (DPG), which controls access to the British prime minister's Downing Street residence, and to Scotland Yard, the police headquarters in London. It also guards embassies in the capital.

"Three officers have been informed there will be no further action regarding criminal matters but they remain subject to a misconduct investigation," a Scotland Yard spokesman said.

That internal probe "will examine whether the conduct allegedly took place whilst the officers were on or off duty".

One of the trio was suspended while the other two were placed on restricted duties after the investigation was launched.

A fourth officer, who was interviewed on Jan 8 in connection with the investigation, was also placed on restricted duties. State prosecutors have yet to decide on criminal charges in his case.

Besides Downing Street, Scotland Yard and foreign embassies, the DPG also protects government ministers, former premiers, some former government ministers, and visiting dignitaries.

The three arrested officers had been held 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 Scotland Yard spokesman said.

The DPG is the same squad caught up in the so-called "Plebgate" row that brought down cabinet minister Andrew Mitchell.

One member was jailed for 12 months on Thursday after he falsely claimed to have witnessed a brief argument at the Downing Street gates between Andrew Mitchell, then the government chief whip, and an officer in Sept 2012.

Mitchell was forced to resign over claims he called the DPG officers "plebs" because they refused to let him go through the main gate with his bicycle.

Mitchell admitted swearing but denied using the word "pleb", a derogatory term for the lower social classes.

Four officers have a case to answer for gross misconduct in relation to the incident, the police have said.

Scotland Yard said the Dec 19 arrests were not linked to the "Plebgate" investigation in any way, but came about following inquiries made as a result of that probe, the BBC reported.