Police used sex as a tool during undercover operations - and tactical "promiscuity", sanctioned by senior commanders, was viewed as "part of the job", a former agent revealed.
The officer, who worked in a secretive unit of the Metropolitan Police for four years, said sexual relationships with activists were common among those gathering intelligence from anarchist, left-wing and environmental groups.
His claims, made to The Observer, contradict comments made last week by the Association of Chief Police Officers (Acpo) which insisted the practise was forbidden.
But the former officer, once a member the Special Demonstration Squad (SDS), a covert unit formed to prevent violent disorder on the streets of London, said sex helped to maintain cover. The man, who was not named, admitted to sleeping with at least two of his female targets for information.
"Everybody knew it was a very promiscuous lifestyle," he told the paper. "You cannot not be promiscuous in those groups. Otherwise you'll stand out straight away."
Meanwhile, there was also no set of instructions dictating whether officers could or could not have sex with activists, he claimed.
"Among fellow undercover officers, there is not really any kudos in the fact that you are shagging other people while deployed," he added.
"Basically it's just regarded as part of the job. It'd be highly unlikely that you were not (having sex). When you are using the tool of sex to maintain your cover or maybe to glean more intelligence - because they certainly talk a lot more, pillow talk - you would be ready to move on if you felt an attachment growing."
However, the officer, who infiltrated anti-racist groups between 1993 and 1997, said falling in love could jeopardise an investigation and was regarded as unprofessional.
His revelations follow the controversy surrounding former Met officer Pc Mark Kennedy who monitored the actions of protesters across Europe under the guise of a long-haired, drop-out climber called Mark Stone.