Unite, the country’s biggest union, said is understood to have secured a further £4m for 97 of the 256 claimants, whose original compensation offers the union deemed inadequate. This brought the total compensation package to £10,435,000.

The union said today that the pay-outs could range from £25,000 up to £200,000 per claimant, depending on such factors as the loss of income and the seriousness of the defamation.

The case follows the discovery in 2009 of a blacklist of building workers held by the Consulting Association. It uncovered a list of more than 3,000 names and reference cards which detailed workers’ political views, competence, trade union membership and industrial relations activities.

The list had been used by dozens of construction firms to vet those applying for work on building sites.

The firms involved were Balfour Beatty, Carillion, Costain, Kier, Laing O’Rourke, Sir Robert McAlpine, Skanska and Vinci.

A separate settlement was reached by the GMB and UCATT unions, along with law firm Guney, Clark and Ryan (GCR), last month but the amount could not be announced until the Unite case was settled.

The GMB said it understood the total compensation for blacklisting was about £75m for 771 claimants, with legal costs on both sides estimated at £25m.

Analysis of the GMB, UCATT and GCR settlements show that Scotland has the highest regional pay-out figure of over £1,665,000 to 31 workers. The area with the second highest settlement for blacklisted workers was Greenock in Inverclyde with £405,000 paid out to 6 workers. Other Scottish town and cities with blacklisted workers include Glasgow, Renfrew, Androssan, Irvine, Gourock, Grangemouth, Port Glasgow, Hamilton, Livingston, Edinburgh, Aberdeen, Cowdenbeath, Falkirk, Parkhall and West Kilbride.

Unite general secretary Len McCluskey said the scale of the damages underlined the “gravity of the misdeeds” of the companies involved.

He said: “The sums to be paid out go a considerable way to acknowledge the hurt, suffering and loss of income our members and their families have been through over many years.”

Howard Beckett, the union’s legal services director, said the companies involved “had to be dragged kicking and screaming to make unprecedented admissions of guilt” in October last year.

“In addition to financial compensation, admissions of guilt and formal apologies, the companies have agreed, as a result of this litigation, to issue guidance to site managers to ensure blacklisting is not occurring on a local level,” he said.

Tim Roache, GMB general secretary, said the companies involved thought they were above the law: “Finally they have been held to account in public and at great cost to them financially and reputationally. Government and employers’ organisations must never forget this sordid episode.”

Maria Ludkin, the union’s legal director, said it had won the best settlements possible from the blacklisters for GMB members.

“To the bitter end, the companies have remained in denial that they were blacklisters, fearful that public acknowledgement could cost them public sector contracts worth billions of pounds,” she said.

The firms said the settlement with Unite brought to a close all the claims in the litigation.

“These construction companies wish to draw a line under this matter and continue to work together with the trade unions at national, regional and site level to ensure that the modern UK construction industry provides the highest standards of employment and HR practice for its workforce,” they said in a joint statement.