An internal police investigation has uncovered credible evidence that an undercover officer deceived a 19-year-old woman into a long-term sexual relationship.

Andy Coles has denied that he had an intimate relationship with the woman while he infiltrated political groups in the 90s, dismissing her claims as “lurid”.

Now a Scotland Yard investigation has ruled in favour of the woman after she made a complaint alleging that he had deceived her into a sexual relationship.

The inquiry said Coles would have faced a disciplinary hearing on a charge of gross misconduct if he had not already retired from the Metropolitan police in 2013. An officer found guilty of gross misconduct is likely to be sacked.

The woman, known only as Jessica, is also taking legal action against the Met, alleging that Coles “groomed and manipulated” her. She said she was “naive, idealistic, unsophisticated and a very young 19” when she started a sexual relationship with him.

She did not know that Coles, then 32, was a police spy when they had a relationship lasting more than a year. She says he was her first proper boyfriend. The Met recently told her it was no longer contesting her legal action and would discuss compensation.

Three years ago, Coles resigned as the deputy police and crime commissioner for Cambridgeshire and Peterborough after his undercover past was revealed. He said media coverage had “significantly impacted his ability” to conduct his duties.

Andy Coles when undercover in 1991. Photograph: Noor Admani/Arrow

However, he has remained in another post, as a Conservative councillor on Peterborough city council. Jessica and others have called for him to step down as a councillor.

His covert work came to light after an apparent indiscretion by his younger brother, the broadcaster and former pop star Richard Coles, who had made a fleeting reference to his brother going undercover in an autobiography published in 2014.

Richard Coles, who was in the band the Communards before becoming a priest, had written that his brother had infiltrated “some sinister organisation while his wife and baby daughter made do with unpredictable visits”.

The Undercover Research Group, a network of activists examining the work of police spies, received a tipoff about the reference to his brother. The group and the Guardian uncovered how Andy Coles infiltrated animal rights groups between 1991 and 1995.

Jessica, an animal rights activist, has previously said: “Although not legally underage, I feel that my youth and vulnerability were used to target me. I was groomed by someone much older, and far more experienced, and I was manipulated into having a sexual relationship with him.”

In legal papers, she said she would not have consented to the relationship, which started in 1992, if she had known he was a police officer.

The Met’s directorate of professional standards started its investigation in 2017 after she lodged her complaint. In its letter outlining the results of its investigation, the Met said Coles was twice interviewed under caution and on both occasions gave a prepared statement denying he had an inappropriate relationship and declined to answer any other questions.

In 2018, he gave a statement to his local paper saying he denied Jessica’s claims, which he called “lurid”.

In the letter, the Met told Jessica that her complaint had been upheld, adding: “In all of the circumstances, it is credible that Coles met you during his deployment and … a relationship developed.”

A judge-led public inquiry that is examining the covert infiltration of political groups since 1968 is due to examine the conduct of a large number of undercover officers including Coles.

Coles did not respond when the Guardian asked if he wished to comment. In its letter, the Met said its investigation had not found him guilty of gross misconduct, adding that if he had still been a police officer, “he would have been subject of a misconduct hearing where the evidence would be presented and a finding made”.

Jessica said: “He is totally unfit to be in public office and a position of power as he abused it in the worst possible way when he was an undercover officer.”