An undercover British police officer had a two-year intimate relationship with a woman he was spying on, but his superiors did nothing to stop it, according to media reports based on police documents submitted for a human rights probe.

Mr Mark Kennedy was so close to environment and social justice activist Kate Wilson that he moved in with her, visited her parents often and attended functions of her extended family.

But Mr Kennedy, who pretended to be an activist and called himself Mark Stone, was one of more than 140 undercover officers known to have been deployed by the British police to infiltrate political groups since 1968.

A document released last Friday by Police Spies Out Of Lives, the organisation that provides legal support to Ms Wilson and other women who were deceived by undercover cops, said legal papers submitted by the police admit that "an as yet unknown number of cover officers and a line manager knew about and acquiesced" to Mr Kennedy's relationship with Ms Wilson.

The police made their submission ahead of an Oct 3 hearing of Ms Wilson's case against the Metropolitan Police and the National Police Chiefs' Council by the Investigatory Powers Tribunal, a court that examines allegations of human rights abuse by the state.

The acknowledgement appears to contradict an unprecedented public apology made by the Metropolitan Police in 2015, as part of their settlement of a civil case brought by eight women, including Ms Wilson, who had been deceived by undercover officers.

Scotland Yard said then that such relationships were "abusive, deceitful, manipulative and wrong" and would "never be authorised in advance".

Ms Wilson was the only member of the group who chose not to settle for compensation but fought on in an attempt to discover more about what had happened.

She won a High Court battle against the Metropolitan Police in 2016 after they withdrew from the case.

She started to speak out under her real name and has since become a cause celebre in her fight to seek justice from the police, alleging that they had violated her human rights.

"It has taken me eight painful years to discover that managing officers really did conspire to deceive and abuse me, something the police had consistently denied," Ms Wilson said in a statement last Friday.

"The wide questions for society here are massive, this is about institutional sexism, senior police officers sanctioning sexual abuse, and the systematic violation of political beliefs, and we still don't have the whole truth."

Ms Wilson spoke of how she fell in love with Mr Kennedy at a 2003 meeting to plan a protest against the Group of Eight industrialised nations' summit in Scotland in 2005.

"He was charming and disarming," she wrote. "He shared my interests and my passion for the political things that we were doing.

"He told me lots of his most intimate stories and secrets. We became very close. We spent two years living together as lovers.

"He visited my parents on many occasions, and he attended my grandmother's 90th birthday.

"He was my partner in just about everything for two years."

It was not until 2010 that she realised she had been duped.

Through a friend, she learnt that he was an undercover police officer, that his name was not Mark Stone, and that he was married with children. She was one of a number of women he had romantic relationships with during his seven years infiltrating environment activist groups.

The Metropolitan Police would say only on Friday that it "has made clear its position on long-term sexual relationships known to have been entered into by some undercover officers in the past". It added: "Those relationships were wrong and should not have happened."