Thousands of employees at Marks & Spencer are demanding answers after being told they may be at risk of identity fraud following the theft of a company laptop.

The computer, which stored the details of more than 20,000 workers, was snatched from the premises of a printing firm in a burglary on April 18. The company was given the laptop so that it could print letters to M&S employees regarding pension changes.

Executives at the retail giant have began writing to 26,000 members of staff in its final salary pension scheme warning they are in danger if the information is accessed by identity criminals.

The computer is believed to contain the employees’ salary details, addresses, dates of birth, national insurance and phone numbers, according to a report on BBC Radio 4.

M&S is offering free credit checks to all those who are affected.

“The weekend brings news of another trusted high street brand losing sensitive information,” said Jamie Cowper, data encryption expert at PGP Corporation. “Laptops, it seems, will continue to go missing. Companies need to take responsibility for their customers’ and employees’ personal data and protect it with sensible security measures.”

M&S could not be reached for comment.