Scotland Yard has admitted Special Branch officers passed information to a controversial network that blacklisted construction workers.
It follows a six-year battle to find out if the Metropolitan Police supplied the intelligence on trade unionists.
The force says its investigation had “proven” the allegation, which will be investigated by a public inquiry.
Workers who say they were unfairly barred from jobs have already received millions of pounds in compensation.
In 2016, the union Unite reached a settlement with construction firms that resulted in 256 workers sharing more than £10m in compensation.
At the heart of the claims, which were made by hundreds of workers, was evidence that firms accessed a “blacklist” that logged workers’ trade union activities.The list was used by dozens of construction firms to vet those applying for work on building sites.