Woman whose partner was an undercover cop with a wife and children reveals how she found out

Discovering that your partner is being unfaithful can be one of the most devastating moments of your life, but for one British activist in a five-year relationship with a supposed fellow protester, this heartbreak and betrayal turned into horror.

That is because she not only found out that her long-term boyfriend had a secret wife and family, but that he was actually an undercover cop who started a relationship with her to infiltrate activist groups in the UK.

Often referred to as the spy cops scandal over the pond, the woman who goes by the name Alison was just one of dozens who were targeted by undercover police units to gather intelligence on left-wing groups in Britain. Only three far-right groups were targeted.

The English school teacher was a young anti-fascist and anti-racist when she first met Mark Cassidy, who introduced himself fluently in radical-speak as an 'anarcho-syndicalist'. But it was not until she looked in his coat pocket years later that her suspicions began.

The UK held a major public inquiry into the undercover cops scandal that saw the units closed (Peter Macdiarmid/Getty Images)

After seeing her supposed boyfriend spending an increased amount of time outside of their shared home, Alison told ITV's This Morning that she had given into her suspicion and intuition and decided to look in his jacket pocket.

Alison shared: "I thought there was something he was keeping from me, I didn't think it was this. It doesn't reflect that well, but I looked in his jacket that he'd left behind and I found a bank card. It was in a different name, M. Jenner."

When he came back, she questioned him about the card and he claimed it was stolen and he'd bought it off someone in a pub. While she cut up the card, Alison did not investigate further, adding: "I'm just a teacher, what do I know about buying stolen credit cards off guys in pubs?"

It was only when 'Mark Cassidy' finally walked out of her life after five years together, suddenly citing 'depression', that Alison began to realize something was up.

Alison told the programme: "When he left, with a note on the table, that's not how you leave a five-year relationship. Not in my world."

Alison shared her heartbreaking story of falling in love with someone who was not who they claimed to be (ITV/This Morning)

This prompted her to track down people who knew him, including the one living family member that her ex had admitted to.

"It became apparent that he didn't want to be found," she added.

After setting up a campaign group with other women who suspected they had been in relationships with undercover agents, Alison eventually got into contact with her ex's wife, who detailed that every time he left Alison's home he had gone to hers, and vice versa.

Mark - whose real name was Mark Jenner - also secretly had children while in a relationship with Alison. Last year, he was called to give evidence as part of the undercover policing inquiry. He said he felt it was necessary to have sex with women as part of his surveillance, but it was 'definitely not' a perk of the job, the BBC reported.

15 years later, Alison reflected: "It's not about him, he was part of a machine. He had managers. There was a back office. He had a manual that told him how to manage this deceit.

"This wasn't about prosecuting people, this wasn't about putting people through the criminal justice system, this was simply about surveilling."

"It's akin to the Stasi," Alison added, comparing the operation to the East German secret police.

"I'm not the only one, what we know is that there are more than 60 women that this has happened to."

A lengthy inquiry into this scandal in British policing is expected to conclude next year after more than 10 years of evidence gathering and questioning the undercover cops who perpetuated relationships with women in activist circles.

The undercover units directed officers to infiltrate Greenpeace, the Campaign for Nuclear Disarmament, and a number of socialist groups. They have since been disbanded.