An undercover officer who had an unauthorised sexual relationship with an environmental campaigner has been dismissed from the police after a disciplinary hearing.

Jim Boyling, who infiltrated leftwing groups for five years, was found guilty of gross misconduct. He is the first undercover officer sent to spy on political groups who is known to have been dismissed for sexual misconduct.

The campaigner, who wishes to remain anonymous and is known as Laura, said: “This is an important verdict today, not only to protect the public from Boyling taking such roles in future but also to send a message out far and wide to other officers that such conduct is totally unacceptable.”

On Thursday, the Metropolitan police said Boyling’s conduct had been unacceptable as the force announced the verdict of the four-day disciplinary hearing which had been held in private.

Boyling, who did not attend the hearing and has denied wrongdoing, has previously criticised the Met for picking on him and seeking to justify the large amount of taxpayers’ money spent on investigating the covert infiltration of political groups.

The undercover officer, who had been a detective constable in the Met’s counter-terrorism command, is also facing a legal attempt to reverse a decision not to prosecute him for deceiving another woman into a sexual relationship during his covert deployment.

Police chiefs have claimed that undercover officers were not permitted to form relationships with campaigners they were assigned to spy on. But the police spies did so frequently. Some have left the police and are not subject to disciplinary proceedings, while the Met declines to say whether others still in the police have been disciplined.

The misconduct panel was held seven years after Boyling was exposed as an undercover officer when Laura disclosed details of their relationship to the Guardian.

Boyling used a fake identity to infiltrate environmental and animal rights groups between 1995 and 2000 as a member of a Met covert unit, the special Demonstration squad.

During his deployment, he had sexual relationships with three campaigners. He started an intimate relationship with Laura in 1999 without telling her his true identity. The following year he ended the relationship, saying he was having a breakdown and needed to travel overseas.

Laura investigated his background and discovered that much of it was untrue, causing her to become increasingly frightened and paranoid.

She saw him in London after more than a year when he admitted he had been a police spy. He said he regretted what he had done and that he still loved her.

Laura has described how she was desperately vulnerable and fearful at the time and how he isolated her and exploited her state of mind to entrap her deeper in what she said was an “abusive, manipulative, controlling” relationship.

They married and had two children before separating in 2008.

The Met has apologised and paid compensation to Laura and other women after admitting that undercover police had deceived them into “abusive, deceitful, manipulative” relationships.

The Met said the disciplinary panel had ruled that Boyling had had a long-term sexual relationship using his fake identity, had concealed this relationship from his superiors and disclosed police secrets.

Boyling’s lawyers said he had been told that he had been dismissed and did not wish to comment. In a statement this week, Boyling said he had told the Met about his relationship with Laura: “The Met does a good line in selective amnesia as indeed they do in selective disclosure.”

He added: “The position of the Met appears to be that a relationship entered into as an operational tactic is acceptable, but a genuine one resulting in marriage and children constitutes misconduct.”

Deputy assistant commissioner Richard Martin said: “The Met recognises that cases such as these demonstrate there were failures of supervision and management.”