The names of thousands of construction workers were kept on a secret list and sold to companies for an annual fee, a court has heard.
Ian Kerr, 66, of Avoncroft Road, Stoke Heath, Worcestershire, admitted compiling the database containing personal information and work details.
The Information Commissioners Office said the details on more than 3,200 people were offered to about 40 firms.
A solicitor entered Kerr's guilty plea at Macclesfield Magistrates Court.
The case was remitted to crown court for sentencing but no date has been set.
The court heard Kerr's organisation, the Consulting Association, in Droitwich, West Midlands, was raided by officers from the Information Commissioner's Office (ICO) last March and he was charged with a breach of the Data Protection Act.
The organisation charged firms a £3,000 annual subscription fee and £2.20 to access an individual's details on its database.
Assistant Information Commissioner Mick Gorrell said the practice was a "flagrant abuse" of people's rights and by its covert nature meant those who were on the database were unaware of its existence and powerless to challenge its content, accuracy or fairness.
He said: "Many people will have been left without a job and with no idea why."
The ICO would consider taking enforcement action against companies that had subscribed to the list, Mr Gorrell added.
It said Amec Construction, Balfour Beatty and Morgan Est were among the 40 subscribers.