Britain's most senior police officer has said he cannot be sure that undercover officers are not still getting involved in sexual relationships with partners who do not know their real identities.

Eleven women are taking legal action against the Metropolitan police over claims that they were duped into relationships in the past with undercover officers and had suffered emotional trauma after discovering the deception.

Police chiefs and ministers have given contradictory answers about whether undercover officers were authorised to form sexual relationships with people they had been sent to spy on.

The Guardian has published evidence showing that undercover officers routinely started sexual relationships with political campaigners during an infiltration campaign that began in 1968.

On Thursday, Sir Bernard Hogan-Howe, the commissioner of the Metropolitan police, said his force had guidelines that said police spies should not get involved in sexual relationships, but that the rules could not prevent "human beings sometimes failing".

He told the London Assembly police and crime committee: "We have a policy that says that our officers should not engage in sexual activity with targets or anyone else they meet, or a victim, while they are serving as a police officer.

"We think there is a need for transparency in this area. It is highly sensitive. Any policy cannot prevent human beings sometimes failing. What we need to know is, if that should happen, an individual does have sexual activity, that their manager knows and we react to that."

Under questioning from committee member Jenny Jones, Hogan-Howe said he could not be sure that officers had not continued to get involved in intimate relationships during his two years as commissioner.

"Our policy says it shouldn't, but can I be absolutely confident that it's never happened during my time as commissioner? I can't say that.

"What I can tell you is that we've got things in place via supervision and monitoring to make our best attempt to make it clear to our officers that it shouldn't, and if it does, to tell us.

"I'm confident of that, but I cannot be absolutely sure that there has not been the odd digression, or that there won't be in the future. But I do know that there's less chance of it now than there was 20 years ago."

The women who are taking legal action against the Metropolitan police had long-term intimate relationships with undercover officers between the 1980s and 2010.

After the meeting, Jones said: "It is completely unacceptable for undercover police to have sex with the people they are targeting. Other countries legislate to make clear this practice is unacceptable and it's time for the UK to do the same."