Workers who complained of safety violations were reported by police spies and prevented from getting another job in the construction industry, an investigation has uncovered.

Richard Martin of the Metropolitan Police admitted that allegations workers were put on a ‘blacklist’ used by building firms to vet staff had been “proven”.

It came in response to a complaint filed by the Blacklist Support Group (BSG) to a police regulator in 2012.

Deputy assistant commissioner Martin said in a letter to the BSG: “Allegation: Police, including Special Branches, supplied information that appeared on the Blacklist, funded by the country’s major construction firms.

“The report concludes that, on the balance of probabilities, the allegation that the police or Special Branches supplied information is ‘proven’.”

Despite the admission, no further action will be taken against the officers involved until a public inquiry has concluded, the Met said in a statement.

The blacklisting scandal erupted in 2009 after a government regulator dispatched investigators to raid offices belonging to The Consulting Association in Droitwich, Worcestershire. Inside, a handwritten database containing information on 3,212 workers was discovered. It had been used by over 40 construction companies to vet prospective staff.