He was a burly, funny scouser called Mark Cassidy. His girlfriend – a secondary school teacher he shared a flat with for four years – believed they were almost "man and wife". Then, in 2000, as the couple were discussing plans for the future, Cassidy suddenly vanished, never to be seen again.

An investigation by the Guardian has established that his real name is Mark Jenner. He was an undercover police officer in the Metropolitan police's special demonstration squad (SDS), one of two units that specialised in infiltrating protest groups.

His girlfriend, whose story can be told for the first time as her evidence to a parliamentary inquiry is made public, said living with a police spy has had an "enormous impact" on her life.

"It has impacted seriously on my ability to trust, and that has impacted on my current relationship and other subsequent relationships," she said, adopting the pseudonym Alison. "It has also distorted my perceptions of love and my perceptions of sex."

Alison is one of four women to testify to the House of Commons home affairs select committee last month.

Another woman said she had been psychologically traumatised after discovering that the father of her child, who she thought had disappeared, was Bob Lambert, a police spy who vanished from her life in the late 1980s.

A third woman, speaking publicly for the first time about her six-year relationship with Mark Kennedy, a police officer who infiltrated environmental protest groups, said: "You could ... imagine that your phone might be tapped or that somebody might look at your emails, but to know that there was somebody in your bed for six years, that somebody was involved in your family life to such a degree, that was an absolute shock."

Their moving testimony led the committee to declare that undercover operations have had a "terrible impact" on the lives of innocent women.

The MPs are so troubled about the treatment of the women – as well as the "ghoulish" practice in which undercover police adopted the identities of dead children – that they have called for an urgent clean-up of the laws governing covert surveillance operations.

Jenner infiltrated leftwing political groups from 1994 to 2000, pretending to be a joiner interested in radical politics. For much of his deployment, he was under the command of Lambert, who was by then promoted to head of operations of the SDS.

While posing as Cassidy, he could be coarse but also irreverent and funny. The undercover officer saw himself as something of a poet. A touch over 6ft, he had a broad neck, large shoulders and exuded a tough, working-class quality.

By the spring of 1995, Jenner began a relationship with Alison and soon moved into her flat. "We lived together as what I would describe as man and wife," she said. "He was completely integrated into my life for five years."

Jenner met her relatives, who trusted him as her long-term partner. He accompanied Alison to her mother's second wedding. "He is in my mother's wedding photograph," she said. Family videos of her nephew's and niece's birthdays show Jenner teasing his girlfriend fondly. Others record him telling her late grandmother about his fictionalised family background.

Alison, a peaceful campaigner involved in leftwing political causes, believes she inadvertently provided the man she knew as Mark Cassidy with "an excellent cover story", helping persuade other activists he was a genuine person.

"People trusted me, people knew that I was who I said I was, and people believed, therefore, that he must be who he said he was because he was welcomed into my family," she said.

It was not unusual for undercover operatives working for the SDS or its sister squad, the national public order unit, to have sexual relationships with women they were spying on. Of the 11 undercover police officers publicly identified, nine had intimate sexual relations with activists. Most were long-term, meaningful relationships with women who believed they were in a loving partnership.

Usually these spies were told to spend at least one or two days a week off-duty, when they would change clothes and return to their real lives. However, Jenner, who had a wife, appears to have lived more or less permanently with Alison, rarely leaving their shared flat in London.

It was an arrangement that caused personal problems for the Jenners. At one stage, he is known to have attended counselling to repair his relationship with his wife. Bizarrely, at about the same time, he was also consulting a second relationship counsellor with Alison.

"I met him when I was 29," she said. "It was the time when I wanted to have children, and for the last 18 months of our relationship he went to relationship counselling with me about the fact that I wanted children and he did not."

Jenner disentangled himself from the deployment in 2000, disappearing suddenly from Alison's flat after months pretending to suffer from depression.

The police spy left her a note which read: "We want different things. I can't cope ... When I said I loved you, I meant it, but I can't do it." He claimed he was going to Germany to look for work.

It was all standard procedure for the SDS. Some operatives ended their deployments by pretending to have a breakdown and vanishing, supposedly to go abroad, sending a few letters to their girlfriends with foreign postmarks.

Alison was left heartbroken and paranoid, feeling that she was losing her mind. She spent more than a decade investigating Jenner's background, hiring a private detective to try to track him down. She had no idea he was actually working a few miles away at Scotland Yard, where he is understood to still work as a police officer today.

The strongest clue to Jenner's real identity came from an incident she recalled from years earlier when he was still living with her. "I discovered he made an error with a credit card about a year and a half into our relationship," she said. "It was in the name Jenner and I asked him what it was and he told me he bought it off a man in a pub and he had never used it. He asked me to promise to never tell anyone."

The Metropolitan police refused to comment on whether Jenner was a police spy. "We are not prepared to confirm or deny the deployment of individuals on specific operations," it said.

Alison told MPs that the "betrayal and humiliation" she suffered was beyond normal. "This is not about just a lying boyfriend or a boyfriend who has cheated on you," she said. "It is about a fictional character who was created by the state and funded by taxpayers' money. The experience has left me with many, many unanswered questions, and one of those that comes back is: how much of the relationship was real?"

• If you have information about Mark Jenner or any other undercover police officers contact: paul.lewis@theguardian.com