An undercover police officer who deceived a woman into a five-year relationship and strung her along when she wanted to have children with him had “sadistic tendencies”, the spycops inquiry has heard.

Mark Jenner lived with the woman, known as Alison, as a couple without telling her that he was an undercover officer sent to spy on political campaigners.

He then disappeared from her life after feigning a nervous breakdown. She discovered his true identity only after years of her own detective work.

Giving evidence on Monday, Alison, a leftwing activist, said Jenner had shown “sadistic tendencies” in his treatment of her that left her devastated.

He was given a commendation by his superiors for his covert deployment.

Alison testified at the inquiry, which is examining how about 139 undercover police officers spied on thousands of predominantly leftwing campaigners between 1968 and at least 2010.

One of the central issues in front of the inquiry is how over decades the undercover officers routinely formed intimate relationships with women while concealing their real identity from them. At least 50 women are known to have been deceived by the police spies.

Jenner infiltrated anti-fascist and leftwing campaigns, including a group exposing police corruption, between 1995 and 2000. Soon after he started his covert deployment, he began a relationship with Alison and then moved into her home in Hackney, London. He lived with her for four years.

The inquiry heard that Jenner has claimed that his motivation for the long relationship was that Alison had “a landline telephone and knew a lot of people in Hackney, so her home made an ideal base for him to continue his undercover operation”.

She said this claim was “complete nonsense and ridiculous”, adding: “Everybody had a landline phone in 1995.” She said it was “just not true” that he was “in constant demand” through the landline. “There’s a whole web of lies around where Mark Jenner was living during these years,” she said.

She described how he was “completely integrated” into the fabric of her life as she considered him to be her trusted long-term partner. He attended close family gatherings, such as weddings and a funeral. She went on holidays with him, including to Israel, Vietnam and Thailand.

She did not know that throughout their relationship he was married to another woman with children.

By 1998, Alison, then in her mid-30s, wanted children with him. He said he wanted them as well, but later. After he closed down conversations on the subject, she persuaded him to go to counselling for more than a year to resolve the matter.

She said he strung her along, claiming that he was not ready yet. This revealed “a sadistic streak”, she said. “It is unfathomable … I can’t wrap my head around the idea that anyone could do this.”

In 2000, Jenner disappeared from her life after months of pretending to have a breakdown – a regular ruse deployed by the undercover officers to end their deployment without arousing the suspicions of those they were infiltrating.

She said this was “absolutely shameful” and “manipulative” as she was concerned that he may commit suicide.

She soon suspected that he was some sort of state agent and spent thousands of hours carrying out “obsessive” research during some “very dark” years, she said. A decade later, she had established that Jenner had been a police spy.