At least 25 undercover police officers who infiltrated political groups formed sexual relationships with members of the public without disclosing their true identity to them, the Guardian can disclose.

The total shows how women were deceived on a systemic basis over more than three decades. It equates to nearly a fifth of all the police spies who were sent to infiltrate political movements.

One woman, known as Jacqui, has said her life was “absolutely ruined” after she discovered by chance that the father of her son was an undercover officer, more than 20 years after his birth. The officer, Bob Lambert, abandoned them when the son was an infant, claiming falsely that he had to go on the run abroad to escape being arrested by police.

Other women had intimate relationships lasting up to six years with men who concealed the fact they were undercover officers who had been sent to spy on them and their friends.