Retired judge Sir John Mitting, the inquiry's chair, is examining the conduct of 139 undercover officers who spied on more than 1,000 mainly left-wing groups.

Male police spies were later found to have formed sexual relationships, and even fathered children, with female activists who were unaware of their true identity.

The Metropolitan Police has apologised for the "enormous distress that has been caused" and said undercover operations had gone through "radical reform".

However, it refused to confirm or deny whether disruptive protest groups were still being infiltrated today.

The interim report published on Thursday looks at the period between 1968 when the Special Operations Squad (later renamed the Special Demonstration Squad) was formed, up until 1982.

It finds that some methods used, including the use of dead children's identities, "would have been bound to have given rise to legitimate public concern and to embarrassment to the commissioner and to his police authority - the home secretary".

The report also states: "Long term deployments into political groups inevitably required the undercover officer, male or female, to befriend members of the target groups and to enter into their personal and political lives.

"Putting to one side the risk that sexual relationships might develop, this intrusion into the lives of many hundreds of people in this era required cogent justification before it should have been contemplated as a police tactic."

Sir John says: "None of these issues appears to have been addressed by senior officers with the MPS (Metropolitan Police Service) or by Home Office officials during this period."

He says a report in 1976 conducted by senior Met police officers into the operations concluded that the work of undercover officers was of "extreme importance" in helping to police public order functions.

However, he finds that issues around the methods used were not examined.

While it is clear the government knew about these operations, what doesn't emerge from the report is who at the highest level knew and signed off the tactics that Sir John says would have led to them being shut down.

Undercover unit 'would have been shut down'

Sir John says: "If these issues had been addressed, it is hard to see how any conclusion could legitimately have been reached which would not have resulted in the closure of the SDS (Special Demonstration Squad)."

The report accepts that long-term infiltration of political single-issue groups could be justified "if its purpose was to prevent or investigate serious crime, including terrorist activity", and notes such groups existed during the Cold War era and the 'Troubles' in Northern Ireland.

In this first phase of the report, mostly covering the 1970s, it suggests three groups fell into this category, two not identified to the public in 'closed' evidence and the other being "(Provisional) Sinn Fein".

However, it finds "the great majority of deployments by the SDS in this period did not satisfy either criterion".

The principal purpose of infiltrating left-wing and anarchist groups was to control public order.

Under the Heath government (1970-74) the main concern was industrial unrest, and under Callaghan (1976-79) it was the infiltration of trade unions by the Communist Party of Great Britain and of the Labour Party by Militant Tendency.

While the report finds undercover policing did "make a real contribution", it finds the same thing could have been achieved by "less intrusive means".

Campaigners will have to wait another three years before the full findings are published, extending over a much longer period, to at least 2010.

But speaking after the first tranche today, 'Jessica' - who was tricked into a sexual relationship - said in a statement that the report revealed that SDS officers had used "racist, offensive and sexist language".

"[It] shows the contempt with which they held campaigners - they had no guardrails, whether reporting on children or making salacious comments on people's sexual activities," she said.

Dave Smith, a trade unionist who was spied on, said thousands of construction workers had been blacklisted by employers as a result of police infiltrating meetings and picket lines.

He said it showed more wrongdoing by the Met and that "anyone involved in genuine civic society - trade unionists, environmental campaigners, political parties, that are perfectly legal" were seen as a legitimate target for surveillance.

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In his "work in progress", inquiry chair Sir John states: "Some issues are better addressed when all of the evidence about them is in, notably the impact of the conduct of male police officers on women deceived into sexual relationships with them, and on the families of the officers.

"The impact on the surviving relatives of deceased children of the adoption of their identity; and the purpose of gathering intelligence on 'justice' campaigns.

"For the same reason, I have also refrained from expressing any general conclusions about the attitude of police officers and managers within the unit towards deceitful sexual relationships during deployments."

Met admits 'legacy of hurt'

This will be a disappointment to campaigners already frustrated at the delays in the inquiry - launched in 2015 by then home secretary Theresa May and originally expected to conclude in 2018.

At the launch of this interim report, journalists were told many of the concerns - such as the impact on women by the conduct of male officers and the tactic of using dead children's names - become "bigger issues in later years".

Met Police said the report acknowledged some undercover work had been valuable in preventing disorder and that most officers performed their duties "conscientiously and in the belief that what they were doing was lawful".

However, it conceded a "legacy of hurt" had been caused by the "unacceptable and immoral behaviour" of some officers.

Commander Jon Savell said in a statement that he wanted to "reiterate the apologies made to women deceived by officers into sexual relationships, to the families of deceased children whose identities were used by officers, and to those who suffered a miscarriage of justice because of the actions of SDS officers".

He said undercover policing had been transformed "with greater regulation, professional codes of practice, and judicial oversight" and "bears no relation" to the 1970s.

But when questioned by Sky News he repeatedly refused to say if infiltration operations are still being used today in regards to protest groups such as Extinction Rebellion.

Mr Savell insisted undercover work was a legitimate tactic that "protects people from crime", adding: "I appreciate that people are going to want to know what is going on right now, but of course it wouldn't be appropriate for me to confirm or deny exactly what is happening right now because it would compromise it."