The ruling last Friday came at the end of a two-day hearing where it emerged that documents had been destroyed relating to firms’ involvement with the Consulting Association blacklisting body.

Firms and four individuals will now have to carry out costly searches of back-up computer tapes of emails to disclose any relevant information by 12 February.

The court also ruled that contractors must pay costs for the hearing, estimated at up to £100,000.

Information that surfaces will be taken into account in calculating damages for the blacklisted workers when the case goes to full trial in May. Another pre-hearing is set to take place on February 1.

Last October eight major firms admitted collating information and vetting workers secretly in the long-running case, which is being backed by construction union Unite on behalf of 168 workers.

Unite director of legal services Howard Beckett said: “The ruling is a major step on the long road to justice for blacklisted workers many of whom have been tormented by questions about why they were targeted and their livelihoods destroyed.

“Despite admitting their guilt, it is shameful the lengths that some of the construction firms involved in blacklisting have gone to cover up their involvement.

“It is only now after sustained legal action with the support of Unite that the lid is being lifted on a scandal which has ruined countless lives and led to hardship for many more.”