In only the second prosecution of its kind in the UK, investigating environmental health officers (EHOs) discovered that an employee at the company's Edinburgh store had been allowed to work for more than 68 hours in a week - 20 hours over the 48-hour limit.
The employee worked for more than 12 hours a day on occasion, and received no additional payment or recompense.
A spokesman for shopworkers' union Usdaw said the prosecution would serve as a stark reminder to all employers. "What it does is put employers on notice that if they ask their staff to work illegal hours they will be punished," he said.
He warned: "Tired staff make more mistakes, are less productive and are more prone to absenteeism."
Martin McColl declined to comment.