The police or security services supplied information to a blacklist funded by the country's major construction firms that has kept thousands of people out of work over the past three decades.
The Information Commissioner's Office (ICO) has revealed that records that could only have come from the police or MI5 have been discovered in a vast database of files held on 3,200 victims who were deemed leftwing or troublesome.
The files were collected by the Consulting Association, a clandestine organisation funded by major names in the construction industry.
Its database was seized nearly three years ago, but the extraordinary nature of the information held has only now emerged, following an employment tribunal for one of the victims, Dave Smith, a 46-year-old engineer who had a 36-page file against his name and was victimised repeatedly for highlighting safety hazards on sites, including the presence of asbestos.
David Clancy, investigations manager at the ICO, told the central London tribunal adjudicating on Smith's claims against construction giant Carillion that "there is information on the Consulting Association files that I believe could only be supplied by the police or the security services".
Speaking to the Observer, Clancy added: "The information was so specific and it contained in effect operational information that wouldn't have formed anything other than a police record."