Police have admitted for the first time that an undercover officer had a sexual relationship with an environmental activist with the knowledge of bosses.
Legal documents seen by the BBC reveal they knew about Mark Kennedy's relationship with Kate Wilson and allowed it to continue.
She was among several women he had relationships with while undercover.
In public statements to date, police have maintained such relationships would never have been sanctioned.
Posing as an environmental campaigner, Mark Stone, the police spy began a two-year relationship with Ms Wilson in 2003.
She was one of a number of women he was involved with during his seven years undercover. In his real life, Mr Kennedy was married with children.
Ms Wilson is currently involved in legal action against the Metropolitan Police and the National Police Chiefs Council.
At different times the Met and the NPCC's predecessor, the Association of Chief Police Officers, were responsible for Mr Kennedy's deployments.
In documents disclosed to her legal team, the police concede that Mr Kennedy's sexual relationship with Ms Wilson "was carried out with the acquiescence of his cover officers and line manager".
Ms Wilson told the BBC: "The police have said that these cases were a failure of supervision and management, and that is just not the case.
"Management were absolutely complicit in what was going on."
In November 2015, the Met Police paid compensation to a number of women who were duped into relationships with police spies.
In an apology at that time, the force said: "The forming of a sexual relationship by an undercover officer would never be authorised in advance."
Ms Wilson's lawyer, Harriet Wistrich said: "The police have always said this would never be permitted.
"What we are seeing in this case - and then of course it makes us question all the other cases - is that they allowed it to continue because it was convenient, because it assisted whatever their objectives were.
"That is incredibly disturbing."
In the legal documents, the police admit that the relationship contravened Ms Wilson's human rights and the breach was made worse because Mr Kennedy's bosses knew what he was doing.
The police admissions are part of their response to a case Ms Wilson is pursuing through the Investigatory Powers Tribunal, which hears claims arising from the Human Rights Act. A hearing will take place on 3 October.
Ms Wistrich said: "Kate was involved in social justice and environmental campaigning.
"She does not expect that the state could actually allow an undercover officer to have a sexual relationship in order to facilitate his gathering of intelligence.
"It is a very shocking revelation in a so-called democratic society."
The Met said it would not comment on the revelations because of the ongoing legal action.
"The Metropolitan Police Service has made clear its position on long-term sexual relationships known to have been entered into by some undercover officers in the past.
"Those relationships were wrong and should not have happened."