Warehouse workers at Sports Direct, the retail chain controlled by billionaire Mike Ashley, appear to be receiving pay below the national minimum wage, according to expert analysis of a new Guardian undercover investigation. For two weeks in June and July, an undercover reporter recorded how staff at the Shirebrook, Derbyshire, were unable to leave the warehouse during their 30-minute unpaid breaks – a practice some employment law experts say should count as paid working time and, if correct, would push Shirebrook’s, in effect, hourly wage rates below the legal minimum of £8.72 to about £8.20. Frasers Group, the holding company for Sports Direct, said that staff did not have to perform any duties or be at the company’s disposal during rest breaks. They said workers were allowed to use their breaks freely and for their own purposes. The concerns have emerged almost five years after the Guardian first exposed how the retailer was breaching minimum wage law, which resulted in workers receiving about £1m in back pay.