Marks & Spencer bugs its staff, according to the whistleblower who was fired after leaking details of the retailer's redundancy plans.
Tony Goode, a former customer loyalty manager for the chain, told delegates at the Trade Unions Congress (TUC) staff were constantly monitored and were afraid to speak out.
Mr Goode claims his personal phone calls from his mobile may have been monitored by the company.
The GMB union, which is representing Mr Goode in his appeal, said M&S staff at the company's headquarters are under constant surveillance from CCTV.
The union also said the company monitored every computer key stroke made by staff, then kept the information for six years.
Mr Goode, 43, lost his job after leaking details of M&S's redundancy plan to a newspaper.
He claims his concerns were ignored by the company and he had to speak out.
However, M&S has dismissed his claims of surveillance as "utter nonsense".
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