Campaigners stormed the London offices of Sir Robert McAlpine yesterday to perform to citizen's arrest on the blacklist boss Cullum McAlpine.

The stunt was timed to coincide with the fifth anniversary of the raid by the Information Commissioner's Office that discovered the illegal building industry blacklist.

Eyewitnesses reported scuffles between Blacklist Support Group members and McAlpine staff before police arrived.

When appearing in front of a select committee Cullum McAlpine admitted he had been the first chairman of the blacklisting organisation the Consulting Association.

He also revealed that the association's first meetings were held at a Sir Robert McAlpine Limited property.

Blacklist Support Group secretary Dave Smith said: "Blacklisted workers have lost their houses and our kids were on free school meals while Cullum McApline is the lord of the manor in his £4.5million Grade I Listed Mansion in Cold Ashton."

The association's database held files on 3,213 individuals that included information on trade union membership and records of when they had flagged up health and safety concerns on building sites.

Workers who appeared on the blacklist suffered years of unemployment, repeated dismissals and financial hardship.

During the building of the Olympics Stadium in 2008-9, Sir Robert McAlpine was invoiced in excess of £28,000 for blacklisting name checks.

The Independent Police Complaints Commission is investigating police involvement in blacklisting and has already confirmed that Special Branch "routinely provided information about prospective employees."

Blacklist activist Helen Steele said: "It has been five years since this conspiracy between big business and the police was discovered.

"Trade unionists and environmental activists have had their human rights violated. But, five years later, no-one has been brought to justice for the crimes they have committed."

Sir Robert McAlpine and seven other major construction firms involved in blacklisting are joint defendants in a High Court conspiracy case scheduled for April.