The Tradetrak figures from ACNielsen Homescan are based on value sales of core grocery lines, and do not include non-food sales.
With that in mind, the 0.2% slip in year-on-year market share for the latest quarter recorded by both Somerfield and Kwik Save stores underlines the challenge facing the group.
Somerfield reported like-for-like sales growth of 1% in the year to April 26 and unveiled plans for delivering its renewal programme with "urgency and pace".
Executive chairman John von Spreckelsen told the City that the group was seeing good returns from its investment in store refits and product development. And he added: "We expect sales growth to gather momentum as this investment continues."
Morrisons and Waitrose show it is possible to focus on grocery and grow market share they both enjoyed a 0.1% rise in the latest quarter compared with the same period last year.
But it is the performance of Tesco and Asda that really impresses. Given the amount of attention being paid to their work in key non food areas, the fact that both retailers are still managing to grow their market share in grocery is worth noting. Asda saw its market share rise by 0.4% year- on-year, while Tesco was up 0.5% year-on-year.
The biggest losers during the quarter ending June 14 were Sainsbury and Safeway. Both companies saw a 0.7% fall in their share of expenditure on groceries.