Undercover police officers routinely adopted a tactic of "promiscuity" with the blessing of senior commanders, according to a former agent who worked in a secretive unit of the Metropolitan police for four years.

The former undercover policeman claims that sexual relationships with activists were sanctioned for both men and women officers infiltrating anarchist, leftwing and environmental groups.

Sex was a tool to help officers blend in, the officer claimed, and was widely used as a technique to glean intelligence. His comments contradict claims last week from the Association of Chief Police Officers that operatives were absolutely forbidden to sleep with activists.

The one stipulation, according to the officer from the Special Demonstration Squad (SDS), a secret unit formed to prevent violent disorder on the streets of London, was that falling in love was considered highly unprofessional because it might compromise an investigation. He said undercover officers, particularly those infiltrating environmental and leftwing groups, viewed having sex with a large number of partners "as part of the job".

"Everybody knew it was a very promiscuous lifestyle," said the former officer, who first revealed his life as an undercover agent to the Observer last year. "You cannot not be promiscuous in those groups. Otherwise you'll stand out straightaway."

The claims follow the unmasking of undercover PC Mark Kennedy, who had sexual relationships with several women during the seven years he spent infiltrating a ring of environmental activists. Another two covert officers have been named in the past fortnight who also had sex with the protesters they were sent to spy on, fuelling allegations that senior officers had authorised sleeping around as a legitimate means of gathering intelligence.

However Jon Murphy, Acpo's spokesman on serious and organised crime, said last week that undercover officers were not permitted "under any circumstances" to sleep with protesters.

He added: "It is grossly unprofessional. It is a diversion from what they are there to do."

Mounting anger among women protesters will see female activists converge on Scotland Yard tomorrow to demand that the Met disclose the true extent of undercover policing. The demonstration is also, according to organisers, designed to express "solidarity with all the women who have been exploited by men they thought they could trust".

Climate campaigner Sophie Stephens, 27, who knew Kennedy, said there was fury among women who felt violated by the state: "We know women have been abused by men posing as policemen and it's becoming clear this was state-sanctioned. These women did not know they were forming a relationship with policemen. It's appalling – and now we want the full details of the undercover officers to be made public."

The protest will be followed on Tuesday by the appearance before the Commons home affairs select committee of the acting Met commissioner, Tim Godwin, and Commander Bob Broadhurst, who is responsible for public order in the capital. Both will be asked to explain why Scotland Yard gave false information over the use of covert operatives during the London G20 protests in 2009. The issue of sexual activity by operatives is also likely to be brought up.

The former SDS officer claims a lack of guidelines meant sex was an ideal way to maintain cover. He admitted sleeping with at least two of his female targets as a way of obtaining intelligence.

"When you are on an undercover unit you were not given a set of instructions saying you could or couldn't do the following. They didn't say to you that you couldn't go out and drink because technically you're a police officer, that you shouldn't go out and get involved in violent confrontations, you shouldn't take recreational drugs.

"As regards being with women in very, very, very promiscuous groups such as the eco-wing, environmental movement, leftwing, or the Animal Liberation Front – it's an extremely promiscuous lifestyle and you cannot not be promiscuous in there.

"Among fellow undercover officers, there is not really any kudos in the fact that you are shagging other people while deployed. 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.

"The best way of stopping any liaison getting too heavy was to shag somebody else. It's amazing how women don't like you going to bed with someone else," said the officer, whose undercover deployment infiltrating anti-racist groups lasted from 1993 to 1997. Two years later the SDS became the National Public Order Intelligence Unit, the secretive organisation that employed Kennedy and whose activities are the subject of three investigations.

The officer added that undercover police were strictly encouraged not to form a bond with women they were sleeping with and said that he knew Jim Boyling, the undercover officer who married an activist he was supposed to be spying upon.

Boyling, a specialist operations detective constable with the Met, was suspended on Friday pending an investigation into his professional conduct.

The former SDS officer, who has now left the Met, said one stipulation by senior commanders was that undercover officers should be married, so that they had something to return to. He said the move was introduced when a spy never returned after five years undercover.