Dozens of companies admitted referring to a blacklist held by The Consulting Association (TCA) to see whether or not workers were part of a trade union or had raised health and safety concerns on sites.

The eight companies in question - Balfour Beatty, Carillion, Costain, Kier, Laing O'Rourke, Sir Robert McAlpine, Skanska UK and Vinci - are now working with trade unions recognising they have no option but to negotiate a compensation scheme for the thousands of workers affected.

Richard Arthur of Thompsons Solicitors, said: “The Westminster Government's cavalier attitude towards privacy when it comes to six million trade union members and their willingness to go to any lengths to protect the bonuses of their donors in the banking sector was recently highlighted around the Trade Union Admin Bill currently before parliament.

“The fact that it has taken years for eight construction firms to be dragged kicking and screaming to concede that they were wrong to use TCA and wrong to blacklist is a text book example of why we need to ensure the privacy of trade union membership data.

"We hope that significant compensation is awarded to all of those who have been affected, and support is given to get blacklisted workers back into work.”