The privacy watchdog has shut down a West Midlands consultancy for selling personal information on prospective workers to the construction industry.
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The Information Commissioner's Office (ICO) seized a database in a raid at the Consulting Association in Droitwich.
Consulting Association owner Ian Kerr appears to have run the database over 15 years and charged construction firms a £3,000 annual subscription fee, the ICO said.
Ian Kerr was allegedly charged for personal details held about individuals on lists of potential employees submitted by subscribers.
Prominent construction firms Balfour Beatty, Sir Robert McAlpine, Taylor Woodrow and Laing O'Rourke are listed among Kerr's past and present subscribers.
David Smith, deputy information commissioner, said the database represents a serious breach of the Data Protection Act.
"Not only was personal information held on individuals without their knowledge or consent, but the existence of the database was repeatedly denied," he said.
The ICO has served an enforcement notice on Kerr, who now faces prosecution for breaching the Data Protection Act (DPA).
The DPA requires UK organisations to be open about how they process personal information and register with the ICO, which Kerr failed to do.
Smith said the ICO is also considering taking regulatory action against the construction firms that have been using the illegal database to vet workers.