Undercover police officers were today given the green light to start sexual relationships with suspected criminals if it helps them avoid being found out.

Policing minister Nick Herbert said this was essential otherwise suspects could use the offer of sex as a way of outing police if they feared they were under surveillance.

Mr Herbert’s comments follow the controversy over Met officer Mark Kennedy who infiltrated environmental protesters opposed to a power station in Nottinghamshire.

He had affairs with two of the activists he was monitoring and the trial of six campaigners later collapsed after details of his role, which had initially been kept secret from defence lawyers, were disclosed.

The case led to a review of undercover policing by the Inspectorate of Constabulary and widespread calls for greater checks and scrutiny of operations.

Mr Herbert made clear today that sex was still permitted under the Regulation of Investigatory Powers Act, which stipulated that officers could break rules normally barring them from having sex in “very limited circumstances”.

Speaking in a debate in Westminster Hall, he added: “Banning such actions would provide the group targeted the opportunity to find out whether there was an undercover officer within their group.” Mr Herbert said “specific statutory guidance on the circumstances of sexual relationships” was unnecessary, but that surveillance operations should be strictly monitored to prevent the law being abused.

At least four more officers — including at least two who fathered children — are alleged to have had sex with women they were monitoring.

Eight women who claim they were duped into long-term relationships of up to nine years with five officers have now started legal action. One of the officers who fathered a child is former Met officer Bob Lambert who infiltrated animal rights and environmental groups in the Eighties.

He has admitted conning a woman into having an 18-month relationship with him, apparently so that he could convince activists he was genuine. Another undercover police officer, Peter Black, said when he was deployed sex was used to gain the trust of activists.

Kennedy, who posed as eco-warrior Mark “Flash” Stone, has denied duping three women into sex. “I never conned anyone into having sex,” he said. “I lived undercover for eight years and if I hadn’t had sex, I would have blown my cover. But I never used these women to gain information. The love we had was real.”