One of the country's largest building companies was allegedly charged more than £2,000 a month by a private detective who supplied personal details about thousands of workers.

Sir Robert McAlpine is one of more than 40 companies accused by Richard Thomas, the Information Comissioner, of breaching data protection laws after an investigation by his office sparked fears that many workers were being unfairly “blacklisted”.

The commissioner alleges that Ian Kerr, of Droitwich, Worcestershire, charged companies £3,000 a year to consult his database of 3,213 workers, whose names were accompanied by notes such as “poor timekeeper, will cause trouble” and “Irish ex-army bad egg”.

Invoices obtained by The Times appear to indicate that Sir Robert McAlpine was billed £26,841.20 by Mr Kerr last year for access to the secret database. Documents compiled by the Information Commissioner appear to show that the company paid £5,951 for information on workers between April and June last year.

“It would appear that Sir Robert McAlpine and other companies made thousands of requests a year,” a spokesman for the Information Commissioner said. He said that each request for information on a specific employee cost about £2.20 on top of the yearly subscription. That suggests that Sir Robert McAlpine made more than 10,000 requests last year alone.

Mr Kerr will be prosecuted under the Data Protection Act, which prohibits holding personal information on individuals without their knowledge or consent, the Information Commissioner said. He could face an unlimited fine if he is successfully prosecuted in the Crown Court.

Union leaders said that the Government had passed a law in 1999 outlawing the practice of so-called blacklisting but decided not to implement it on the ground that “there was no hard evidence that blacklisting was occurring”.

Alan Ritchie, the general secretary of Ucatt, the construction union, said: “Ucatt members know from bitter experience of being refused work that blacklisting exists in construction.

“However, the extent of the practice and the fact that most of the major companies are involved in the practice is truly shocking. It is outrageous that construction workers have been barred from jobs simply for being trade unionists.”

Sir Robert McAlpine was unavailable for comment yesterday.

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