All the main chains posted declining sales in the 12 weeks to March 30 - a period distorted by the later timing of Easter this year, latest till-roll figures from Kantar Worldpanel show .
The share held by market leader Tesco fell to 28.6% from 29.7% a year earlier and the 28.7% reported in the survey a month ago. Asda produced the most resilient performance among the big four in the period, with a sales fall of 0.5% leaving its market share at 17.4% against 17.6% a year ago.
In contrast, Aldi achieved its highest growth in sales yet of 35.3% as its market position jumped from 3.4% a year ago to 4.6%. Lidl rose to a record 3.4% from 2.9% as sales improved by 17.2%.
Kantar added that Waitrose held on to its record 5% share, while the Co-op appears to have stemmed recent losses by stabilising at 6.1%.
The share held by Sainsbury's dropped to 16.5% from 16.9% a year earlier after sales fell 1.7%. Morrisons fell to 11.1% from 11.6% a year ago.
Overall grocery sales grew by just 0.6% in the 12 weeks, although Kantar said adjusting for Easter this figure was still low by historical standards at 1.5%.
Kantar Worldpanel director Edward Garner said: "All of the 'big four' supermarkets have faced declining sales over the past 12 weeks, which has been accentuated by the late falling of Easter.
"Nevertheless, they have also seen worrying share declines, with the most resilient performance coming from Asda this period."