The retailer has launched an investigation after Usdaw said foreign agency workers had been working two eight-hour shifts in a 24-hour period, breaching working hours regulations. In between shifts at the Bracknell fruit, vegetable and chilled foods warehouses, the workers were getting a few hours' sleep in a car park, the union said.
"Usdaw is writing to Waitrose to request a formal meeting with management regarding the alleged breach of work rules," said Usdaw national officer Joanne McGuinness. "Usdaw will seek to find out the details of the situation and work with the company to address any transgression of the rules."
A Waitrose spokesman said the retailer was very concerned about the allegations.
"Waitrose has a strong legacy of commitment to the highest standards of employee welfare," he said. "We expect our employment agency to comply with these high standards in addition to UK and international labour laws.
"We are conducting a thorough investigation into these claims."