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

It comes in 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.

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.

“The Report concludes that, on the balance of probabilities, the allegation that the police or Special Branches supplied information is ‘Proven’.

The letter goes on to explain: “Sections of the policing community throughout the UK had both overt and covert contact with external organization, including the Economic League”

It also conformed an “improper flow of information from Special Branch to external organisations, which ultimately appeared on the Blacklist”.

The blacklist scandal has seen more than £75m in compensation paid to workers by major contractors.

Allegations of police collusion in blacklisting were first made back in 2012 but the claims were strenuously denied by the authorities.

MP John McDonnell said: “It is now abundantly clear that various arms of the state including the Police colluded in the blacklisting process.

“This is one of the hidden scandals of the abuse of civil liberties in our country that needs to be recognised fully and addressed. The people involved need to be brought to book.”

Dave Smith, secretary of the Blacklist Support Group said: “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.”

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.”