A police constable from Scotland Yard's diplomatic protection group (DPG) has been spared jail after he admitted sharing extreme porn on his mobile phone while on duty.

James Addison, 37, was fined £6,000 at Westminster Magistrates' Court after admitting he sent pornographic videos and stills to colleagues using social networking app Whatsapp while working as a protection officer at Downing Street last year.

The elite police squad officer, from Rayleigh in Essex, sent other officers "disgusting and offensive" images that included bizarre sex acts and scenes showing defecation, the court heard.

The images were found on his phone during the investigation into the Plebgate affair involving former government chief whip Andrew Mitchell and DPG officers at the gates of Downing Street, the court heard.

No criminal action was taken against Addison over that matter, but he still faces internal misconduct proceedings.

Wearing a black suit and tie and yellow shirt, he hugged tearful relatives as he left the courtroom a free man, although he is likely to lose his job.

District Judge Howard Riddle told Addison: "Even at a time when the state has retreated to a huge extent in sexual judgment and taste and humour, these images go some way beyond what is legal in this country. These images are very unusual indeed."

Sparing the officer a jail sentence, the judge added: "The situation would be very different indeed if this involved children and similarly if there had been involvement of animals.

"I sentence you on the basis that there were no obviously unwilling participants in the films.

"Perhaps the greatest harm is that at least some of these images were received and sent while you were working and while you were being paid by the public to carry out a very important task on behalf of the public."

The judge also accepted that the images had been sent to "shock and amuse" rather than for profit and that Addison had not been involved in making them.

"I fall short of saying that you are not worthy of wearing a police uniform, that is for somebody else to decide," he said.

The court heard that Addison is facing an internal misconduct investigation and remains suspended.

He pleaded guilty to 11 counts of publishing an obscene article under the Obscene Publications Act last month.

Prosecutor Emma Scheer said officers on the Plebgate investigation found that Addison used Whatsapp to send "images of an extreme pornographic nature and some of an obscene nature".

In total, 15 people received images from him, all of them police officers, between February and June last year.

"Some were sent whilst he was on duty and on some occasions they were sent to multiple recipients at the same time," she told the court.

"In terms of the images, some were likely to result in the infliction of serious harm to at least one of those involved and others featured defecation."

James Hines, for the defence, said in mitigation that the images had been sent as a joke rather than for sexual gratification.

Mr Hines said of his client: "He hasn't lost his job but he undoubtedly will do so, there is no doubt about that.

"He is a man of previous good character - a good, solid, hard-working officer and family man," he said.

In total 14 officers have been subject to criminal investigation over possession of and distribution of obscene images on mobile phones.

Four officers from the DPG, including Addison, were arrested but only he was charged.

No further action was taken against the other three in relation to the criminal allegations but they have been the subject of a misconduct probe with eight other officers from the DPG, and two officers based in boroughs.