Sexual activity between undercover police officers and members of the public who did not know their true identities was “not uncommon” from the mid-1970s, a public inquiry has heard.

David Barr QC told the Undercover Policing Inquiry (UCPI) that some officers in the then all-male Metropolitan Police Special Demonstration Squad (SDS) would make jokes about intimate relationships in front of managers.

No written instructions have been found governing sexual relationships by officers in the shadowy unit, but the inquiry heard that there is evidence that, between 1972 and 1983, at least five had intimate contact with as many as 12 women.

Mr Barr said: “It can safely be said that, from the mid-1970s onwards, sexual contact between SDS officers in their undercover identities and members of the public was not uncommon.”

One of the women, known as Mary, became involved with “ladies’ man” Richard Clark, and said: “Had I known he was a police officer there is absolutely no way I would have had any sexual contact with him at all.”

Another officer, known as Vince Miller, claimed he had four one-night stands with different women, but one of these, Madeleine, said the relationship lasted a couple of months and she was “very upset” when he disappeared.

A third, who used the name Jim Pickford, was with his second wife when he went undercover, but met a woman during his deployment who he went on to marry and have a child with.

Mr Barr said there was some evidence that officers had been warned off having intimate relationships, but “we will also be hearing evidence that there was comment and joking amongst SDS undercover officers about sexual relationships that, it is stated, would have been made in the presence of managers.”

The latest series of hearings in the Undercover Policing Inquiry began on Wednesday, looking at the SDS between 1972 and 1983.

This period saw the first time that officers became involved in sexual relationships with members of the public, and were instructed to use aspects of the identities of dead children to shore up their cover names.

Witnesses will include Celia Stubbs, the partner of anti-fascist campaigner Blair Peach, who died when he was hit over the head by a police officer during a protest in Southall, west London, in 1979.

The demonstration came amid tensions arising from the National Front mounting a general election campaign that year.

Mr Barr told the inquiry that the campaign for justice for Mr Peach was described by officers as “the subsequent campaign against the police”.

And in the 1979 annual SDS report, it was stated: “The death of Blair Peach, an active supporter of the Anti-Nazi League, which was a consequence of a violent anti-fascist demonstration in Southall, provided the extreme left-wing with an opportunity to mount a sustained campaign to discredit and criticise the police.”

The SDS maintained that reporting on the campaign allowed uniformed officers to be deployed to locations where public unrest may occur.

Mr Barr said: “We note the defensive language to describe the Blair Peach justice campaign and the fact that reporting on it was communicated to the Home Office as having been part of an invaluable service.

“We shall need to examine the motives for reporting on the campaign.”

Earlier, the Metropolitan Police repeated apologies for officers having sexual relationships and using the identities of dead children without their families’ consent.

In a statement released by the Met on Wednesday, Helen Ball, Assistant Commissioner for Professionalism, said the period included rioting and the start of the IRA bombing campaign in England.

She said: “It was against this challenging backdrop that the SDS were operating.

“In this part of the inquiry, evidence will also be heard about officers’ actions and behaviour, which in some instances were clearly inappropriate and unacceptable – certainly by modern standards, and in some cases by the standards of the time in which they occurred.

“The inquiry will hear examples of undercover officers entering into inappropriate sexual relationships with women they met during their deployments and of undercover officers using the identities of deceased children – a practice that does not happen now.

“The Met acknowledges that these cases caused significant harm and distress, and for this we are sorry.”

The Undercover Policing Inquiry (UCPI) was set up in 2015 to look at the activities of two shadowy police units after condemnation of undercover tactics.

A public outcry was sparked when it was revealed that women had been tricked into sexual relationships with undercover officers and that police spies had used the identities of dead children without their families’ permission.

Family justice campaigns, including for murdered teenager Stephen Lawrence, were spied upon; and there are claims that some officers were arrested or prosecuted for crimes under fake identities, leading to alleged miscarriages of justice for their co-defendants.

The two units being examined are the Metropolitan Police’s Special Demonstration Squad (SDS), which existed between 1968 and 2008, and the undercover part of the National Public Order Intelligence Unit (NPOIU), which existed between 1999 and 2010.

During the current batch of hearings, being chaired by Sir John Mitting, the deployment of 29 undercover officers will be examined, who on average were on assignment for three to five years.

To date the mammoth inquiry has cost more than £36 million, although Tory peer Lord Moylan estimated last week that this could rise to £100 million, including police costs, by the time the inquiry reports in 2023.

The inquiry was adjourned until Thursday.