Two undercover police officers secretly fathered children with political campaigners they had been sent to spy on and later disappeared completely from the lives of their offspring, the Guardian can reveal.

In both cases, the children have grown up not knowing that their biological fathers – whom they have not seen in decades – were police officers who had adopted fake identities to infiltrate activist groups. Both men have concealed their true identities from the children's mothers for many years.

One of the spies was Bob Lambert, who has already admitted that he tricked a second woman into having a long-term relationship with him, as part of an intricate attempt to bolster his credibility as a committed campaigner.

The second police spy followed the progress of his child and the child's mother by reading confidential police reports which tracked the mother's political activities and life.

The disclosures are likely to intensify the controversy over the long-running police operation to infiltrate and sabotage protest groups.

Police chiefs claim that undercover officers are strictly forbidden from having sexual relationships with the activists they are spying on, describing the situations as "grossly unprofessional" and "morally wrong".

But that claim has been undermined as many of the officers who have been unmasked have admitted to, or have been accused of, having sex with the targets of their surveillance.

Last month eight women who say they were duped into forming long-term intimate relationships of up to nine years with five undercover policemen started unprecedented legal action. They say they have suffered immense emotional trauma and pain over the relationships, which spanned the period from 1987 to 2010.

Until now it was not known that police had secretly fathered children while living undercover. One of them is Lambert, who adopted a fake persona to infiltrate animal rights and environmental groups in the 1980s.

After he was unmasked in October, he admitted that as "Bob Robinson" he had conned an innocent woman into having an 18-month relationship with him, apparently so that he could convince activists he was a real person. She is one of the women taking the legal action against police chiefs.

Now the Guardian can reveal that in the mid-1980s, just a year into his deployment, Lambert fathered a boy with another woman, who was one of the activists he had been sent to spy on.

The son lived with his mother during the early years of his life as his parents' relationship did not last long. During that time, Lambert was in regular contact with the infant, fitting visits to him around his clandestine duties.

After two years, the mother married another man and both of them took responsibility for raising the child. Lambert says the woman was keen that he give up his legal right to maintaining contact with his son and cut him out of her new life. He says the agreement was reached amicably and he has not seen or heard of the mother or their son since then.

Lambert did not tell her or the child that he was a police spy as he needed to conceal his real identity from the political activists he was spying on. The Guardian is not naming the woman or the child to protect their privacy.

Lambert was married during his secret mission, which continued until 1988.

The highly secretive operation to monitor and disrupt political activists, which has been running for four decades, has come under mounting scrutiny since last year following revelations over the activities of Mark Kennedy, the undercover police officer who went rogue after burying himself deep in the environmental movement for seven years.

Police chiefs and prosecutors have set up 12 inquiries over the past year to examine allegations of misconduct involving police spies, but all of them have been held behind closed doors. There have been continuing calls, including from the former director of public prosecutions Ken Macdonald, for a proper public inquiry.

The second case involves an undercover policeman who was sent to spy on activists some years ago. He had a short-lived relationship with a political activist which produced a child.

He concealed his real identity from the activist and child as he was under strict orders to keep secret his undercover work from her and the other activists in the group he infiltrated. He then disappeared, apparently after his superiors ended his deployment. Afterwards, she remained under surveillance as she continued to be politically active, while he carried on with his police career.

The Guardian understands that as he had access to the official monitoring reports, he regularly read details of her life with a close interest. He watched as she grew older and brought up their child as a single parent, according to an individual who is aware of the details of the case.

The policeman has been "haunted" by the experience of having no contact with the child, whom he thought about regularly, according to the individual.

• If you have information please contact paul.lewis@theguardian.com