A police spy married an activist he met while undercover in the environmental protest movement and then went on to have children with her, the Guardian can reveal.

He is the fourth spy now to have been identified as an undercover police officer engaged in the covert surveillance of eco-activists. Three of those spies are accused of having had sexual relationships with the people they were targeting.

The details of the activities of the fourth spy, who is still a serving Metropolitan police officer, emerged as the senior police officer managing the crisis in undercover operations insisted that officers were strictly banned from having sexual relationships with their targets.

Jon Murphy, the chief constable of Merseyside, told the Guardian it was "never acceptable" for undercover officers to sleep with people they were targeting.

"Something has gone badly wrong here. We would not be where we are if it had not," he said, referring to three inquiries into undercover policing that have been launched in response to the Guardian's investigation into the first spy, Mark Kennedy, an undercover officer who had several sexual relationships during his seven-year deployment.

Murphy, who is the national lead officer on serious and organised crime for the Association of Chief Police Officers, declined to speak about the Kennedy case directly but said officers who infiltrated the environmental movement were not permitted "under any circumstances" to sleep with activists.

"It is grossly unprofessional. It is a diversion from what they are there to do. It is morally wrong because people have been put there to do a particular task and people have got trust in them," he said.

Meanwhile the ex-wife of the fourth undercover police officer spoke to the Guardian. The woman was married to Jim Boyling, a serving Metropolitan police officer who spent five years living undercover with environmental campaigners between 1995 and 2000.

Using the false identity "Jim Sutton", Boyling infiltrated Reclaim the Streets, an environmental group famed for bringing streets to a standstill in unruly protests against cars.

During his time undercover, when he is said to have become a key organiser, Boyling met a 28-year-old woman and began a relationship with her. He later disappeared from her life.

It was only when he reappeared a year later that he told the woman he was a police officer. They later married and had two children but divorced two years ago.

Speaking for the first time, the woman gave the Guardian a detailed account of their relationship and alleges that Boyling:

• Encouraged her to change her name by deed poll, apparently to conceal their relationship from his seniors at the Met. Her deed poll certificate is signed by Boyling, who lists his occupation as "police officer".

• Told her a ruling from seniors that undercover operatives should not have sex with targets was unrealistic, and developing relationships with activists was "a necessary tool in maintaining cover".

• Only informed a senior officer that he was in a relationship with an activist in 2005, around the time they married using her new identity.

• Named at least two other police officers who served as undercover operatives and indicated other political activists who he believed to be police officers.

Kennedy, who is in hiding in the US, is also believed to have "outed" a fellow spy – an allegation he denies. Police chiefs, who have been unable to establish contact with Kennedy have said any such breach of protocol constitutes "heresy".

Boyling and the Met were given a detailed account of the woman's allegations, but neither provided a response. The woman said tonight she hoped her story would reveal how deep infiltration of the protest movement "wrecks lives". "Everybody knows there are people in the movement who aren't who they say they are," she said. "Being too paranoid would hinder everything. But you don't expect the one person you trust most in the world not to exist." Senior officers say any suggestion they tacitly allowed operatives to have relationships are unjustified, and argue examples of inappropriate behaviour are rare.

Murphy defended the police tactics of infiltrating the environmental movement today. He said the group had a small number in their midst "intent on causing harm, committing crime and on occasions disabling parts of the national critical infrastructure". "That has the potential to deny utilities to hospitals, schools, businesses and your granny," he said.

Senior officers privately admit there was widespread confusion over accountability at the National Public Order Intelligence Unit, which ran both Kennedy and Boyling. "We are left to regulate it ourselves and we think we do a good job of it," said Murphy today. "Sometimes things go wrong, it is a volatile area of police work."

The Guardian also today fully identifies two of the other undercover officers involved in spying on the eco-activists, previously called Officer A and B.

Their names and photographs were not used after representations from senior police, but both have now been extracted from undercover roles in other investigations, and they can be named as Lynn Watson and Mark Jacobs.