Germany's cut-price supermarket chain Lidl was accused yesterday of using Stasi methods to spy on its staff and collect intimate details about their personal lives, including their relationships, bank accounts and the frequency of their lavatory breaks.

The allegations were published in Stern magazine, which said it had obtained hundreds of pages of surveillance reports compiled on Lidl staff in Germany and the Czech Republic by private detectives contracted to spy on employees.

Stern said the information was collected with miniature cameras that were set up in stores with the excuse that they were needed to deter shoplifters. The magazine said the style of the surveillance was almost identical to that used by the former East Germany's notorious Stasi secret police. One excerpt read: "Wednesday 14.05. Mrs M, wants to make a mobile phone call during her break, but she receives a message telling her that she has only got 85 cents left on her prepaid phone account. She finally manages to get in touch with a girlfriend with whom she would like to cook supper, but she insists that her pay must have reached her bank account by then otherwise she won't have any shopping money."

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Another report from a Lidl store in the Czech Republic revealed that women staff members were banned from using the lavatory during their shifts. The only exception were women who were having their period. According to Stern, they were obliged to wear a headband, "visible from a distance", to denote the fact.

Other surveillance report entries included a comment by a detective who made disparaging remarks about a Lidl cashier's "self-made" tattoos. He suggested that she should be asked to cover them up because elderly customers might assume that she got them in prison.

Lidl did not deny the existence of the reports but insisted that they were intended to expose "possible staff misconduct". A spokesman said, however, thatthe allegations concerning the store in the Czech Republic were "not known to us in reality".

Peter Schaar, Germany's data protection officer, described the reports as a "grave infringement" and the state of Baden-Württemberg, where Lidl has its headquarters, said it was beginning an immediate investigation into the allegations under the country's data protection laws.

Achim Neumann, a spokesman for the German shopworkers' union Verdi, said: "This kind of surveillance is conducted by totalitarian states. Germany does not need companies like this."

Allegations that Lidl grossly mistreated its staff first surfaced in 2004 when Verdi published a so-called "Black Book" report based on interviews with Lidl employees. The union alleged that the company routinely interrogated staff and forced them to do unpaid work outside shop hours.

One former Lidl cashier told Verdi: "I did not even have time to go to the toilet. Leaving the till meant being told off, so I sometimes went home with wet underwear."

At the time, Lidl dismissed Verdi's allegations as a "defamation campaign". Lidl has outlets in 23 countries in Europe.