Morrisons' sales fell 1.7% in the 12 weeks to Nov. 3, trailing market leader Tesco, Sainsbury's and Asda, which saw declines of 0.6%, 0.2% and 1.2% respectively, market researcher Kantar said.
In contrast, Lidl's sales rose 8.8% and Aldi's were up 6.7%, giving the discounters a combined market share of 13.9%.
Unlike the big four players, Aldi and Lidl continue to open lots of new stores.
Shares in Morrisons were down 0.4% at 0834 GMT, Tesco was down 2.1% and Sainsbury's was up 0.7%.
Overall UK grocery sales grew 1.0% over the 12 weeks. "The increase is slightly behind the equivalent rate last month, against a backdrop of political uncertainty and a persistently wet autumn," Kantar said.
Grocery inflation was 0.8% over the period, it said. Prices are rising fastest in markets such as fresh sausages, frozen fish and ambient cooking sauces, while falling in fresh poultry, butter and instant coffee.
Walmart-owned Asda is due to update on third quarter sales on Thursday.
(Reporting by James Davey; editing by Kate Holton and Jason Neely)