A secret police document has revealed how the Metropolitan Police's Special Branch helped the illegal blacklisting of trade unionists - preventing them from getting jobs because of their political views.
In one case, detectives suggested one individual was a terrorist, despite the claim being wrong.
The illegal practice - exposed 10 years ago - involved major construction firms accessing secret files on 3,000 workers and their union activities.
But until now, little has been known about the police's role, other than a Scotland Yard admission it had been involved.
Part of the secret report underpinning that admission has now been disclosed, after initially being classified as so secret it was for the Metropolitan Police Commissioner's eyes only.
The report - codenamed Operation Reuben - found "numerous areas of concern" with "inappropriate contact of Special Branch officers with private organisations", including with one of the two blacklisting groups, the Economic League and the Consulting Association.
Blacklisting: How it worked
- Blacklisting began with the Economic League in 1919 which shared records on left-wing activists with industry to keep them out of the workplace
- It was closed in 1993 after a Parliamentary inquiry. The Consulting Association sprang up to replace it
- The Information Commissioner's Office raided The Consulting Association in 2009, revealing for the first time the scale of the operation - triggering legal action that continues to this day
- In 2016, eight major construction firms offered settlements to end legal action: Balfour Beatty, Carillion, Costain, Keir, Lang O'Rourke, Sir Robert McAlpine, Skanska and Vinci
The Reuben investigators found no systematic records of the relationships - but one sharing incident from 1978 had been recorded after a senior officer intervened.
On that occasion, a trade union activist had applied for a job making educational videos with a company linked to the construction industry.
The company passed the individual's name to the Economic League to be checked - which in turn contacted the police for any further intelligence "due to the perceived risk of involvement in education".
"The receiving officer's initial inquiries revealed a potential link to [redacted] which in his opinion had not been resolved satisfactorily... he returned to EL asking for any further information, stressing the matter's importance due to the possible link to terrorism.
"This was recorded as fact by the EL representative."
EL then passed this on to the prospective employer - ending the candidate's chance of getting a job.
The applicant appears to have learned that they had been "blacked by the security people".
One of their relatives was a retired senior police officer who demanded an investigation - and that appears to explain why the incident remained recorded.
One major blacklisting allegation is that an officer called Mark Jenner collected information after he infiltrated the construction union UCATT between 1995 and 2000.
The report says that Jenner, who used the alias Cassidy, provided information on 300 people - and 16 of those appeared in the illegal blacklist database.
Operation Reuben said it found no evidence to prove that Jenner directly provided that intelligence - but it added it could not rule out other officers doing so.
Roy Bentham, joint secretary of Blacklist Support Group, said that many questions remain unanswered.
"The police are supposed to uphold law and order, not spy on perfectly democratic organisations such as trade unions," said Mr Bentham.
"Blacklisting is a national scandal and confirmation that the police colluded with this shameful and unlawful activity is beyond the pale."
Imran Khan QC, lawyer for the Blacklist Support Group, said that the onus was now on the undercover policing inquiry to dig deep.
A spokesman for the Metropolitan Police said that the internal report into blacklisting had established that "certain conduct" amounted to improper sharing of information under the law as it stands today.
"Allegations about police involvement with the 'blacklist' will be fully explored during the Undercover Policing Public Inquiry (UCPI)," said the spokesman.
"The Metropolitan Police Service will await the conclusions of the UCPI before considering any appropriate next steps."