Sainsbury's, reeling from last week's move by the regulator to block its takeover of rival Asda, was alone among Britain's big four grocers in seeing sales fall in the 12 weeks to 21 April, industry data showed on Tuesday.
Sainsbury's sales fell 1.2% in the period, market researcher Kantar said.
In contrast sales at market leader Tesco, Asda and No. 4 Morrisons increased 1.0%, 0.3% and 0.6% respectively.
Kantar said all of the big four lost market share to German owned discounters Aldi and Lidl, whose combined share was 13.6%.
Reclaimed No.2 Position
Sainsbury's did, however, reclaim its position as Britain's No. 2 supermarket group - a status it lost to Asda in Kantar's last data set.
Last Thursday the Competition and Markets Authority (CMA) thwarted Sainsbury's £7.3 billion bid for Walmart owned Asda, saying the deal would raise prices.
Sainsbury's, which is due to publish full-year results on Wednesday, wanted to buy Asda to boost its scale and buying power so it could compete better with Tesco and the discounters.
Kantar said total UK grocery sales rose 2% over the 12 week period, the fastest rate of growth the overall sector has experienced this year.
Grocery inflation was 1.4% over the period, it said.