Police chiefs are taking legal action against one of their former undercover officers who fathered a child during his covert infiltration of leftwing groups and then abandoned him.

The son of the former officer is already suing the Metropolitan police alleging that he has suffered psychiatric damage after discovering at the age of 26 that his father was not a radical protester he claimed to be, but was instead a police spy.

Now it has emerged that the Met is seeking to make Bob Lambert, the former undercover officer, also defend the legal claim that his son has launched.

If the Met succeeds in making Lambert a joint defendant against his son, Lambert could be forced to pay a proportion of any compensation that would have to be paid to his son if he wins his legal claim.

Lambert’s son, known only as TBS as he has been granted anonymity, accused the Met of using “emotional pressure” on him to put him off pursuing his legal claim and getting at the truth.

He was shocked “at the lengths the Met will go to, to avoid saying sorry and meeting with me to discuss how much they have hurt me”.

“By continuing to drag this case out makes me fearful that they don’t actually care about the hurt and damage they have caused to me.”

“To find out his true identity at the age of 26 was a shock and I am slowly coming to terms with it all, although people close to me say I struggle to communicate my true feelings, that I keep it all bottled up.” The Met has previously lost an attempt to have TBS’s legal case dismissed.

The move to make Lambert a co-defendant is due to be heard before a high court judge on Wednesday. It appears to be the first time that the Met has used this tactic when defending legal actions involving women who were deceived into intimate relationships by undercover officers.

In the 1980s, Lambert infiltrated animal rights and environmental groups after he stole the identity of a dead child to develop a false persona pretending to be a leftwing protester.

During his deployment which started in 1984, he had a relationship with a then 22-year-old animal rights activist, known only as Jacqui, without disclosing to her that he was a police spy.

TBS was born in 1985. Lambert and Jacqui lived together during the early years of his life. She has described how he seemed besotted with their son.

In 1988, Lambert claimed that he had to flee abroad to Spain because he feared that police were about to arrest him for crimes committed as an animal rights activist.

This was a fake story that he used to extricate himself from his covert deployment without arousing suspicion. In reality, he returned working for Special Branch in London.

While he had been undercover, he had been married with two children in his real life. However he had concealed this family and his police work from Jacqui and TBS.

After Lambert disappeared, Jacqui was left to bring up TBS on her own. They had no contact with Lambert until she discovered by chance in 2012 that he had been a police spy when she happened to read a story about his undercover work in a newspaper and tracked him down.

The discovery profoundly traumatised Jacqui. In 2014, she was paid more than £400,000 by the Met.

Lambert and the Met did not comment.