Undercover police officers can start sexual relationships with suspected criminals if it means they are more plausible, Home Office Minister and Sussex MP Nick Herbert said today.

He said that under the Regulation of Investigatory Powers Act 2000 (RIPA), officers were permitted to have sex as part of their job but the legislation meant the operations were strictly managed.

There had been confusion about whether undercover police were allowed to go that far following the collapse of a case against environmental activists in Nottinghamshire after it emerged the group was infiltrated by an officer called Mark Kennedy, who had been in sexual relationships with two women in the campaign.

Today, Mr Herbert said it was important police were allowed to have sex with activists because otherwise it could be used as a way of outing potential undercover officers.

Speaking in a debate in Westminster Hall, Mr Herbert said: "In very limited circumstances, authorisation under Ripa Part 2 may render unlawful conduct with the criminal if it is consentutory conduct falling within the Act that the source is authorised to undertake.

"But this would depend on the circumstances of each individual case and consideration should always be given to seeking legal advice.

"I am not persuaded that it would be necessary to introduce specific statutory guidance on the circumstances of sexual relationships under Ripa.

"I think what matters is that there is a general structure and system of proper oversight and control rather than very specific instructions as to what may or may not be permitted.

"Of course, there is another point that banning such actions would provide the group targeted the opportunity to find out whether there was an undercover officer specifically within their group."

But Caroline Lucas, the leader of the Green Party and MP for Brighton Pavilion*, *who is calling for a public inquiry into the conduct of undercover officers, said this meant that, while the police needed a warrant to enter your home, they did not need any authorisation to start a relationship.

She accused the police of failing to take seriously the legal claims of eight women who said their lives had been devastated by the intrusion of undercover police officers.

Ms Lucas said officers had "crossed the line", adding that the case of Mark Kennedy - who went under the alias Mark Stone during his time infiltrating a group that attempted to take over Ratcliffe-on-Soar power station in Nottinghamshire - had "shone a light" on the practices of subterfuge used by the police.

She said: "We need to know what the truth is and we need any rules of engagement to be published and open to public and parliamentary scrutiny or challenge."