A senior United Nations official has called on the British government to launch a judge-led public inquiry into the "shocking" case of Mark Kennedy and other undercover police officers who have been infiltrating protest groups.

Maina Kiai, a UN special rapporteur, said the scandal involving undercover police cultivating intimate sexual relationships with political activists over long periods of time had been as damaging as the phone-hacking controversy that prompted the Leveson inquiry.

He said he was "deeply concerned" about the UK's use of undercover police officers in non-violent groups exercising their democratic rights to protest.

"The case of Mark Kennedy and other undercover officers is shocking as the groups in question were not engaged in criminal activities," Kiai told a central London news conference. "The duration of this infiltration, and the resultant trauma and suspicion it has caused, are unacceptable in a democracy.

"It is a clear violation of basic rights protected under the Human Rights Act, and more generally under international law, such as the right to privacy."

He added: "This is not a James-Bond-type movie issue. I think it is unacceptable that the state can pay somebody who will use women, and be part of their lives and then just devastate them and leave them. That's unbelievable."

Kiai is the latest senior figure to call for a full investigation into the controversy since the Guardian began revealing details of the spy operation two years ago. The undercover policing controversy will be raised in parliament next month during a special hearing hosted by the home affairs select committee.

Undercover police have been living double lives for several years among protest groups, sometimes even residing with female activists and spending weeks abroad with them on holiday. At the end of their deployment, the police spies vanish without a trace.

The surveillance operation, which has continued to plant long-term spies in protest groups despite recent controversies, comes under the remit of an initiative to combat what police call domestic extremism. Many of the targets of the operation have turned out to be law-abiding anti-capitalist campaigners or protesters against global warming.

In at least three cases, relationships between police and the women they were spying on have resulted in the birth of children.

The UN rapporteur's preliminary report follows a 10-day fact-finding mission to London, Belfast and Edinburgh. Kiai met campaigners, senior police, civil servants and the home secretary, Theresa May. He said she told him a full inquiry into undercover policing was "not something on the agenda".

However, Kiai, who has responsibility in the UN for the rights to freedom of peaceful assembly, said he believed the case of Kennedy and others had left a "trail of victims and survivors in its wake" who deserved answers.

Eleven women and one man are bringing a high court legal action for the emotional trauma suffered as a result of "deeply personal" relationships they formed with men who turned out to be police officers.

A judge ruled last week that some of their claims should be heard by the Investigatory Powers Tribunal, an obscure body that usually deals with complaints against MI5 and MI6.

Mr Justice Tugendhat cited the fictional case of James Bond to argue that when parliament introduced legislation allowing covert police to have personal relationships with targets, they must have assumed they may have sexual encounters.

Rejecting the idea that it could be a "James Bond movie issue", Kiai said: "I therefore call on the authorities to undertake a judge-led public inquiry into the Mark Kennedy matter, and other related cases, with a view to giving voice to victims, especially women, who were deliberately deceived by their own government, and paving the way for reparations."

The government has so far resisted calls for a judge-led inquiry, instead choosing to back a host of other separate reviews into the conduct of Kennedy and related issues.

Fifteen inquiries have so far been launched into the controversy since January 2011.

All have been held behind closed doors, a process Kiai said was inadequate because it did not allow victims the opportunity to speak about their concerns.

The largest of the inquiries is being run internally by the Metropolitan police, the force that has overseen the spy operation against protesters since 1968. It has declined to provide any detail about the scope or remit of the inquiry.

Jenny Jones, a Green party member on the London Assembly, welcomed Kiai's intervention. "The hacking of voicemail messages was an invasion of privacy and lead to a judge-led inquiry into the practice," she said. "In contrast, the gross invasion of privacy by the police into the lives and families of 11 women – who were not criminals – will be dealt with in a secretive tribunal which not even the women will be able to attend."