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Isael Hermosillo, a mediator who was negotiating labor contracts for California grocers, was one of 130 terminated in March.

Online grocer Ocado, which sells M&S food on its platform, is unaffected by the problems as it runs on an entirely separate system.

"The grocers are desperately trying to navigate significant cost headwinds, while simultaneously competing with discounters like Aldi and Lidl," she says.

But trade groups for grocers are pushing back on the new law.

After releasing its latest financial figures on Friday, Mr Leighton said the grocer was aiming for its prices to be five to 10% cheaper than its rivals going forward.