Sainsbury's shares have fallen as much as 8% on concern that the supermarket group may unveil weak Christmas trading figures later this week.
Analysts are cautious about Sainsbury's festive trading
With the firm due to detail its festive performance on Thursday, there is some speculation that it has missed internal targets for both sales and profits.
Shares in Marks & Spencer have also fallen as much as 3% ahead of its Christmas update on Wednesday.
Festive retail figures released so far show a mixed High Street picture.
While sales at PC World and Dixons were bad enough for parent company DSG to issue a profit warning, department stores House of Fraser and John Lewis have both reported bumper sales.
Fashion retailer Next also saw poor festive trading, as did furniture seller Land of Leather, which was another retailer to warn on profit.
The only supermarket to so far detail its Christmas sales is Waitrose, which reported a giant 28.5% rise from a year earlier.
However the figure - the only one released - is for overall sales, which includes those from new stores, rather than the more telling like-for-like comparison.
Some analysts estimate that Sainsbury's will report same-store sales growth of 3.6% excluding petrol, below key rivals Tesco and Asda.
Sainsbury's shares ended down 3.6%, or 14.5 pence, at 391p.
Meanwhile, Panmure Gordon analyst Philip Dorgan said that he believed Marks & Spencer would unveil its "worst sales performance for some time".
M&S shares finished the session down 3.7%, or 19.25p, at 498.5p.