The admission follows a campaign by blacklisted workers to prove they were spied on by the police.

The GMB union launched the first high court claims on behalf of those blacklisted in November 2013, more than a year after allegations of police collusion in blacklisting were first made but fervently denied.

The police force said its investigation has now “proven” the allegation.

A letter sent by deputy assistant commissioner Richard Martin in response to a complaint made by the Blacklist Support Group to the Independent Police Complaints Commission confirmed an “improper flow of information from Special Branch to external organisations, which ultimately appeared on the Blacklist”.

Justin Bowden, GMB national secretary, said: “The secret blacklisting of 3,213 construction workers and environmentalists was the greatest employment scandal in 50 years.

“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.

“This admission too raises some serious questions regarding other connected cases.”

Unite assistant general secretary, Gail Cartmail, said: “This is a major breakthrough the police have finally been forced to admit what we already knew that they were knowingly and actively involved in the blacklisting of construction workers.”

The total compensation for blacklisting to date is understood to be around £75 million for more than 770 claimants.