The police and Special Branch infiltrated trade unions and provided information to the construction industry about a blacklist of workers, campaigners have been told.
The revelation is made in a letter in response to a complaint from the Blacklist Support Group to the Independent Police Complaints Commission.
The letter says: "Allegation: Police, including Special Branches, supplied information that appeared on the blacklist, funded by the country's major construction firms, the Consulting Association and/or other agencies, in breach of the Data Protection Act 1998.
"The report concludes that, on the balance of probabilities, the allegation that the police or Special Branches supplied information is 'proven'."
A blacklist of construction workers was exposed in 2009 with the discovery that an organisation called the Consulting Association kept secret files on thousands of trade union members, often for raising concerns about safety on building sites. Workers on the illegal database were denied employment on construction projects.
Millions of pounds has since been paid out in compensation by some of the country's biggest construction firms.
An inquiry is currently being held in the High Court into undercover policing.
Dave Smith, secretary of the Blacklist Support Group, said he had waited six years for the news.
"When we first talked about police collusion in blacklisting, people thought we were conspiracy theorists.
"We were told 'things like that don't happen here'. With this admission from the Met Police, our quest for the truth has been vindicated.
"The police are supposed to detect crime; instead they infiltrated trade unions and provided intelligence to an unlawful corporate conspiracy.
"This is why we need the public inquiry into undercover policing to be open and transparent, in order to get to the truth about how police intelligence was shared with private sector third parties including major companies."