The UK's big four supermarkets all lost market share in the 12 weeks to 19 May, market research company Kantar says, as discounters gained ground.
Tesco's share fell to 27.3% from 27.7% a year ago, while Sainsbury's and Asda had equal shares of 15.2%, after sales fell by 1.7% and 0.2% respectively.
Sales at Morrisons fell 0.4%, giving it a share of 10.4%.
Like-for-like sales were unchanged at leader Tesco and fell at Sainsbury's, Asda and Morrisons.
German discounters Aldi and Lidl continued to record strong growth, with sales up 11.1% and 8.5% respectively, giving them a record combined market share of 13.8%.
Sales at Ocado, Iceland, and Waitrose were also up on a year ago.