More than 40 companies, including some of Britain’s biggest construction firms, are facing legal action over claims that they paid a private detective to vet prospective employees.

Balfour Beatty and Sir Robert McAlpine are among companies accused of paying to access a database of construction workers that listed details of trade union activities and work habits.

Ian Kerr, of Droitwich, Worcestershire, charged firms £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, strong [trade union]”.

He will be prosecuted under the Data Protection Act, which prohibits holding personal information on individuals without their knowledge or consent.

Richard Thomas, the information commissioner, intends to issue enforcement notices against construction companies that appear to have paid Mr Kerr. The notices would prevent companies from trading personal data on penalty of being in contempt of court. The investigation continues.

Most of the companies have declined to respond to the allegations, but some said that they were conducting internal investigations. NG Bailey told The Guardian: “We are taking these claims very seriously and an immediate and thorough investigation into the matter has been launched.”

David Smith, the deputy information commissioner, said: “This is a serious breach of the Data Protection Act. Not only was personal information held on individuals without their knowledge or consent but the very existence of the database was repeatedly denied. The covert system enabled Mr Kerr to unlawfully trade personal information on workers for many years helping the construction industry to vet prospective employees.”

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