Postal workers have been urged to make official requests for any information held about them if they suspect their names were kept on a "blacklist" of thousands of workers.
The secret file of over 3,200 mainly construction workers was uncovered in 2009 after a raid by the Information Commissioner's Office (ICO) on the Consulting Association.
Workers were included on the blacklist for raising health and safety concerns on building sites, or merely for being union activists, and were often denied work for years as a result.
It has recently been revealed that other workers, including firefighters, teachers and postal staff, also had their details included on the blacklist.
Dave Smith, of the Blacklist Support Group, told the annual conference of the Communication Workers Union, that postal workers should contact the ICO if they suspected their names were on the blacklist.
"People in this hall will definitely have been spied on by undercover police," he told the Bournemouth conference.