Ex-Ucatt President and London Regional Secretary John Flavin made the claims in a submission to the latest Scottish Affairs Committee hearing into blacklisting this week.

Flavin said: “Officers and officials of the unions in construction have for many years, supplied information to the special branch and the police about activists in their unions as well as construction employees.”

Flavin said the committee had denied him the chance to present his evidence or publish it on their website.

Union officials from Ucatt, Unite and the GMB were quizzed by the committee on Wednesday during its latest sitting.

Flavin’s claim is the latest twist in the blacklisting scandal which has blighted the industry for years.

The Enquirer has seen an affidavit confirming conversations between a construction union official and a special branch officer.

Another former union official said: “I’m sure that officials have passed on information about people in their own union they didn’t get on with or construction workers they had fallen out with.”

One contractor said: “The employers did wrong over blacklisting and have admitted that.

“But it seems a bit rich for the unions to take a holier-than-though stance if their own people have been involved in all this.”

A Unite spokesman said: “Unite supports a public inquiry into the blacklisting scandal that looks at all the evidence leaving no stone unturned.

“The systematic blacklisting funded and organised by construction contractors lasted many years and it is in the public interest to know the full facts.”