Pc Adam Cox, 31, was working in Parliamentary and Diplomatic Protection when he created an alter ego called Emily Whitehouse to exchange explicit chat with three men online.
After being asked to send them revealing photographs, Cox found indecent images online of a Canadian woman who committed suicide at the age of 21 and passed them off as “Emily”.
Police investigating the online chat raided his home last year, and uncovered a stash of 1,691 indecent and extreme images, with one featuring an infant and others showing children as young as seven.
He told police: “I’m not hoarding images. I have never meant to hurt anyone. I’m not a collector. I’ve not got a secret stash.”
On his Emily persona, Cox said: “It’s me. It’s not me. It’s madness, a way of escaping reality.”
Cox, from Windsor in Berkshire, pleaded guilty to four counts of possession of indecent images – 645 of the most serious category A pictures, 201 category B, 449 category C, and 396 extreme pornographic images of bestiality.
He denied encouraging three men to attempt to get indecent images from “Emily” and the charges were ordered to lie on file.
The Old Bailey heard it was impossible for police to establish if the dead woman in the Emily pictures was 16, 17 or 18 when they were taken.
Judge Mark Dennis QC sentenced Cox to 20 months inprison suspended for two years and 250 hours of unpaid work.
He said: “It should be a matter of enduring shame on his part that he engaged in this offending with complete disregard for his oath and responsibility as a serving police officer.”
Judge Dennis said Cox had pretended to be a teenage girl “for kicks”, adding it was “troubling” that he had yet to come to terms with what it was all about.
Prosecutor Charles Falk said Cox had been working for the Metropolitan Police with responsibility for the security of embassies, Parliament and the royal family.
Mitigating, Nick Yeo said: “He is a man who finds it extremely difficult to articulate his motivation and one can quite understand that because the context is extremely unusual conduct, one might think.”
Cox was made subject to a sexual harm prevention order and sacked following a misconduct review by the Met’s Directorate of Professional Standards (DPS).
Assistant Commissioner Helen Ball said: “It is particularly sad and unacceptable that an officer in Pc Cox’s trusted position would behave in such a discreditable way.
“He was in possession of a very large number of images of young children. Any conviction is discreditable, one of this nature where the behaviour has meant the abuse of the vulnerable is deeply so.
“Dismissal without notice is the appropriate sanction in these circumstances.”
Co-defendants Harry Gibbs, 32, of Stevenage, Hertfordshire, Andrew Monk, 39, of Kettering, Northamptonshire, and Ajai Shridhar, 46, of Ealing, west London, admitted attempting to possess indecent images of children and were each handed a 12-month community order.
Over two months in spring 2016, Monk pestered “Emily” for pictures, particularly ones of her wearing high-heeled shoes. He posed sexually explicit questions, such as: “Are you a moaner or a screamer?”
Supply teacher Gibbs’ chat logs with “Emily” went on between July and September 2015. Even though he believed she was under 18, he tried to set her up on the “Chaturbate” – chat and masturbate – website, the court heard.
He told her she had “real model quality” and advised her that sex was “always big business”.
Shridhar asked “Emily” for photos to “cheer” him up as he chatted with her on Skype in February and March last year.
He told her: “Naughty of me to ask, but have you got any pics where you have to wear your school uniform?”
An NSPCC spokesman said: “Behind every indecent image is a child who has been subjected to the most horrific acts in order for this vile material to be produced.
“As a police officer, Cox would have known that by possessing these awful images he has helped to fuel an industry which feeds off children’s suffering.
“To tackle this growing problem, the NSPCC is calling on tech companies, government and law enforcement agencies to ensure this type of content is taken down quickly when it does appear online but most importantly that it can’t be published in the first instance.”