The unprecedented scale of undercover operations used by police to monitor Britain's political protest movements was laid bare last night after a third police spy was identified by the Guardian.

News of the existence of the 44-year-old male officer comes as regulators prepare two separate official inquiries into the activities of this hitherto secret police surveillance network.

The latest officer, whose identity has been withheld amid fears for his safety in other criminal operations, worked for four years undercover with an anarchist group in Cardiff.

Last night a former girlfriend and fellow activist said she felt "colossally betrayed" by "Officer B". The 29-year-old, who had a relationship with him for three months in the summer of 2008 while he was working undercover, said: "I was doing nothing wrong, I was not breaking the law at all. So for him to come along and lie to us and get that deep into our lives was a colossal, colossal betrayal."

The woman, who did not want to be named, said "Officer B" arrived in Cardiff in 2005, becoming a key member of the 20-strong Anarchist network in the city and "one of her best friends". They had known each for three years before their relationship and she said she did not suspect his true identity until after he left Cardiff in October 2009, claiming he had been offered a job as a gardener on Corfu.

According to the woman Officer B's flat was very empty, with no pictures of friends or family and he rarely talked about his past. "He always said he could not tell his family or friends about us because of the age difference ... if it had been anyone else I would have thought that was strange, but because [he] had been such a good friend for so long it really did not enter my mind that he was anything but a stand-up honest man."

Before he left for Corfu he held a goodbye dinner. His former girlfriend said she kept in touch with him for about a month via email, text message and the occasional postcard. Then the contact dried up.

"At first friends started messaging him asking if he was all right, then when there was no response, a few messaged him to say they were worried he was a spy, but we never heard anything."

The woman said that the experience had rocked her confidence and made her suspicious of other campaigners.

"I am incredibly, incredibly angry," she said. "Obviously to do that to anybody is pretty low, but to do that to someone who trusted you and cared about you and did their best to look after you is just unspeakable. I cannot imagine the kind of person who would lie to someone they were having a relationship with for that long and that seriously ... I strongly suspect that he felt very bad about what he was doing, but that is not an excuse."

The latest developments came as the Independent Police Complaints Commission announced it was widening its inquiry to include the controversy surrounding PC Mark Kennedy, who was the first officer unmasked by the Guardian and who also had sexual relations while undercover.

It is understood a second inquiry is to be launched by Her Majesty's Chief Inspectorate of Constabulary on Monday into whether the undercover surveillance was disproportionate.

Last night it was reported that the trial of six campaigners accused of trying to shutdown a power station at Ratcliffe-on-Soar collapsed because police had withheld secret recordings featuring Kennedy and the activists.

The Times said the Crown Prosecution Service abandoned the trial when it was informed that Nottinghamshire police had suppressed tapes that "fatally undermined the case against the protesters".

More details on the scale of Kennedy's key role in protest movements across Europe emerged yesterday, with allegations that he acted as an agent provocateur in Ireland, Germany and Iceland. It was also revealed that the second undercover agent – "Officer A" – was arrested for glueing herself to the Department for Transport during a protest against Heathrow's expansion in February 2008.

In a twist that will further unnerve senior police officers, it emerged that Kennedy has asked the public relations agent Max Clifford to sell his story.