Somerfield has lost a quarter of its market share in the past 12 months, says an influential City analyst.
Dave McCarthy of Schroder Salomon Smith Barney said the decline was "astonishing".
He added: "New management will be trying to address the issues but it will take a huge effort to stem the downward momentum in this business."
McCarthy based his comments on an analysis of Taylor Nelson Sofres Superpanel till roll data for April, which shows Somerfield's market share falling year on year from 4.6% to 3.9% and its struggling Kwik Save subsidiary down from 5% to 3.2%.
The figures also show that April was disappointing for Tesco after two excellent months, said McCarthy. "Rumours of a slight slowdown in sales appear to be supported by the figures although we still suspect Tesco is outperforming the industry average."
Tesco's market share figure was down 0.8% month on month, but still up 0.4% year on year.
Asda's market share was up 0.9% month on month, proving that the previous period's fall was a rogue figure said McCarthy.
{{NEWS }}
No comments yet