M&S shares, which have lost more than half their value since January, fell sharply yesterday and were down 1p to 254¾p today.
"The tide seems to be against M&S at the moment," said Barclays Wealth analyst William Hobbs. "M&S food is likely to be struggling severely."
While M&S is struggling, business is booming at its cheaper rivals. Nielsen said sales at Iceland surged 14.8% in the four weeks to 9 August while TNS showed sales at Lidl grew by 8.4% and at Aldi by 13.6%.