An environmental campaigner who says she was tricked into a sexual relationship with an undercover police officer is mounting a legal challenge to have him prosecuted.
The woman, known as Monica, will ask judges to overturn prosecutors' decision not to charge the Metropolitan Police's Jim Boyling.
A string of officers from a disbanded unit had affairs while undercover.
DC Boyling is suspended and his lawyers have declined to comment on his behalf.
This week Scotland Yard began a closed disciplinary hearing which could see him sacked.
DC Boyling went on to have two more relationships and his behaviour - and what his managers knew about it - is a key element of a long-delayed public inquiry into alleged abuses in undercover policing.
How did the relationship develop?
Twenty years ago, "Monica" was an environmental campaigner with Reclaim the Streets, a protest group that occupied roads and called for better planning of sustainable transport such as bicycles rather than cars.
Jim Boyling was an officer with the Metropolitan Police's Special Demonstration Squad, a secret unit that sent detectives undercover into protest groups police considered might trigger trouble.
DC Boyling met Monica while infiltrating Reclaim the Streets. They became a couple for six months in 1997.
"I loved him in a way, I really felt strongly for him," Monica told the BBC. "At the time I thought he had genuine feelings for me. But now I look at that and I think actually this man was trained. He was a successful police officer. He was duping us all.
"And I was encouraged to be intimate and sexual with somebody who I would never ever have got involved with if I had known who he was: if I had known his true motives and his true identity."
DC Boyling, whom Monica knew as Jim Sutton, later disappeared in unclear circumstances - a tactic used by all of the officers at the end of their deployment.
Monica only discovered who he really was after he was unmasked along with other officers accused of abuses.
What do the women want?
Scotland Yard has settled cases with a dozen women who sued the police force for being tricked into relationships. In an unprecedented apology, the force said the women had been "deceived"
"These [relationships] should never have happened," said the force's Assistant Commissioner Martin Hewitt. "They were a gross violation of personal dignity and integrity."
But the Crown Prosecution Service has refused to prosecute any of the officers for either sexual offences, or misconduct in public office. Monica appealed that decision under a system called the Victims Right to Review - but the CPS still refused to change its mind. Now she is seeking a full judicial review before High Court judges of the Director of Public Prosecution's handling of the file.
She says that the DPP was wrong not to consider charges, including the unusual historic offence of procuring someone for sex, because she had been the victim of deceit - and therefore never consented to the relationship.
Harriet Wistrich, Monica's lawyer, also argues the CPS should consider whether DC Boyling should be prosecuted on the same legal terms as two women who were convicted of sexual assault by impersonating men.
The Crown Prosecution Service said it would not comment on the legal challenge.
DC Boyling has said he will co-operate with the Undercover Policing Inquiry when it calls to give him evidence, although that could be more than a year off.