Police have admitted passing erroneous information about a trade unionist to a secretive blacklisting operation that resulted in the worker being refused a job, an internal police report has revealed.

The previously secret report discloses that a secretive police unit wrongly suggested in the 1970s that the trade unionist, who is not identified, was possibly involved in terrorism.

The internal report was compiled by a senior police officer in 2016 following longstanding allegations that for years, police shared personal information about political activists with a blacklisting operation that was funded and run by major firms.

The 69-page report “clearly established that on the balance of probabilities”, police had supplied information to the blacklist. The firms used the blacklist to deny work to those they deemed to be troublemakers.

The report acknowledged a “potentially improper flow of information” from Special Branch, a covert police unit that monitored political activists, to “external organisations which ultimately appeared on the blacklist”.

The Metropolitan police asked Mick Creedon, the then chief constable of Derbyshire police, to write the report after blacklisted workers lodged a complaint in 2012.

His report has now been disclosed by the Met to the Blacklist Support Group, which represents blacklisted workers and is campaigning to expose the full extent of collusion between the police and the blacklist.

On Wednesday, John McDonnell, the shadow chancellor, is due to speak at a meeting in parliament about the issue.

The blacklist was closed in March 2009 after an official watchdog ruled that it was unlawful.

Since the late 1960s, large firms had funded the blacklist, which stored confidential files on thousands of trade unionists with details such as their personal background, employment histories, disputes with managers, complaints about health and safety, and suspected political affiliations.

The firms checked these files when potential employees applied for work to screen out those they considered to be undesirable. The workers had no idea why they had been rejected, and many suffered hardship for long periods. Often the files were inaccurate.

In 2016, the firms apologised in court and paid £75m to more than 700 blacklisted workers.

Creedon’s report describes how in 1978 the unnamed trade unionist applied for a job with a television company making educational videos. The firm, also unidentified, checked the name with the Economic League, an organisation which at that time managed the blacklist.

The blacklisters identified the trade unionist as a “leftwing sympathiser” and contacted Special Branch.

Special Branch asked the blacklisters for more information about the trade unionist, “stressing the matter’s importance due to the possible link to terrorism”.

However the Economic League recorded this as a fact and told the prospective employer who rejected the trade unionist for the job.

According to Creedon, the issue was “corrected” in an unspecified way because a relative of the trade unionist was a retired chief superintendent who asked former police colleagues about it. The Met said in a statement that this sharing of information would have been “improper”.

Creedon’s report, which acknowledged “numerous areas of concern”, said there was a distinct possibility that police officers shared information without official authorisation but not systemically.

Dave Smith, spokesman for the blacklisted workers, said it was “laughable” that Creedon had dismissed some specific allegations of collusion between the police and the blacklist. In one case, the fact that three anti-racism activists had been stopped at a protest in 1999 was recorded in the files of the blacklist, but Creedon said there was no evidence the information had been shared outside the police.

Police have had specific units to monitor trade unionists and liaise with companies for decades, according to the report.