A former undercover police officer who infiltrated environmental groups for seven years is under investigation by police for deceiving women into sexual relationships and allegedly leaking secrets.

Police chiefs are conducting a criminal investigation into Mark Kennedy, whose covert deployment was exposed principally by one of the women he had deceived.

The investigation is examining whether Kennedy conducted “inappropriate sexual relationships” and whether he broke the 1989 Official Secrets Act, according to police.

Police chiefs have claimed that undercover officers were not allowed to form sexual relationships with the campaigners they were deployed to spy on. At times, however, police chiefs have displayed a more ambivalent stance and argued that such relationships could be permitted.

Kennedy adopted a fake identity for seven years and pretended to be an environmental and leftwing campaigner in a series of groups such as Climate Camp. During his covert deployment, Kennedy formed a number of intimate relationships with women without telling them that he was a police spy. The longest relationship lasted six years.

Police have admitted that Kennedy deceived four of these women, who were environmental activists, into forming “abusive and manipulative” long-term relationships. The Met subsequently paid them compensation.

Police have also admitted that Kennedy’s managers knew he was deceiving one of the activists, Kate Wilson, into a long-term sexual relationship and allowed it to continue.

Kennedy was one of about 140 undercover police officers in two covert units who spied on more than 1,000 political groups in long-term deployments, usually lasting about five years, since 1968.

After he was unmasked in 2010, Kennedy hired celebrity publicist Max Clifford to sell his story to the media and arrange a series of interviews with journalists. He had by then left the Metropolitan police.

The investigation into Kennedy, codenamed Operation Montrose, started in January 2015 at the request of Scotland Yard.

Investigators have interviewed an unidentified individual under caution and six others as witnesses. The investigation is led by the National Police Chiefs’ Council (NPCC), and conducted by Metropolitan police officers, as part of a wider internal inquiry into Kennedy’s former unit.

Undercover officers in the National Public Order Intelligence Unit infiltrated political movements between 1999 and 2011, when it was shut down following revelations about Kennedy and some of its other members.

So far, 20 undercover officers who worked for the two units are known to have had intimate sexual relationships, some of them lasting years, while using fake identities between the mid-1970s and 2010. Police have paid compensation to at least 12 women who were deceived into these relationships. The other unit, known as the Special Demonstration Squad, operated between 1968 and 2008.

The Crown Prosecution Service examined the cases of a number of the officers and decided in 2014 that none of them would be prosecuted for sexual misconduct – a ruling that it defended last year when it was challenged in the high court. It is unclear why Kennedy is being investigated over his relationships after the CPS made its decision.

The NPCC said it “cannot make a comment at this stage, on the basis that Operation Montrose continues to be a live investigation”. Kennedy declined to comment.