Asda, the British arm of Walmart that agreed to a takeover by rival Sainsbury's, was alone among the country's big four grocers in holding its market share in the latest 12-week period, industry data showed on Tuesday.
Asda's sales rose 1.5% in the 12 weeks to 2 December and it maintained its UK grocery market share at 15.0%, market researcher Kantar Worldpanel said.
In contrast, Sainsbury's sales slipped 0.2% and its share dipped to 16.0% from 16.4% a year ago.
Sainsbury's, Britain's second largest supermarket group, agreed a £7.3 billion ($9.3 billion) takeover of Asda, the number three, in April, a combination that could overtake Tesco as Britain's biggest supermarket group.
Competition and Markets Authority
The deal is currently being investigated by Britain's regulator, the Competition and Markets Authority, which is expected to publish its initial findings next month.
Sainsbury's shares were down 0.8% at 0915 GMT.
Tesco's sales declined 0.1%, while Morrisons' were up 0.5%.
Tesco's shares were down 1%, while Morrisons' were down 2%.
German-owned discounters Aldi and Lidl continued to be the big winners with sales increases of 12.2% and 11.2% respectively, partly reflecting aggressive store opening programmes.
Kantar Worldpanel said overall UK grocery sales rose 2.0% over the 12-week period, its slowest rate since March 2017.
"Consumers are benefiting from falling inflation. It now stands at 1.6% – less than half the rate of inflation in December 2017, when it reached 3.6% – leading to a slowdown in the overall market," said Fraser McKevitt, head of retail and consumer insight at Kantar Worldpanel.
Prices are rising fastest in products such as cola, sparkling wine, and dog food, while falling in fresh pork, ambient cooking sauces, and fruit.