The confirmation comes following a six-year investigation to discover whether Scotland Yard colluded with the controversial network to supply information.
The revelation was made in a letter sent by Deputy Assistant Commissioner, Richard Martin, in response to a complaint from the Blacklist Support Group to the Independent Police Complaints Commission.
The letter states: "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.
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"The report concludes that, on the balance of probabilities, the allegation that the police or Special Branches supplied information is 'proven."
Following the news, union Unite said it was now considering taking fresh legal action.
Assistant General Secretary, Howard Beckett, said: "The latest revelations which have confirmed that the police were both actively spying on construction workers and feeding that information to blacklisters potentially has major implications for blacklisting legal cases.
"Unite will be urgently consulting with our legal experts to identify exactly how we can take appropriate legal action on behalf of our members whose lives were ruined due to the activities of the police."
Justin Bowden, GMB National Secretary, said the Met Police's admission amounts to a "constitutional crisis".
"The secret blacklisting of 3,213 construction workers and environmentalists was the greatest employment scandal in 50 years," he said.
"When in 2013 GMB launched the first high court claims on behalf of those blacklisted there were many in the establishment who said we were paranoid conspiracy theorists.
"Admission by the police that they were directly and deeply involved in denying ordinary working people - who in many cases had done little more than raise health and safety concerns - from work and the chance to support themselves and their families is a constitutional crisis that can only be properly addressed by a full, independent public enquiry as GMB has long maintained."
(LM/MH)
