Half of households shopped at Aldi or Lidl at least once over the summer as the discounters kept up their pressure on the "big four" supermarkets.
Latest till-roll figures from Kantar Worldpanel reveal that supermarket price inflation fell for the 11th quarter in a row to stand at 0.2% in the 12 weeks to August 17, leaving overall sales growth at another 10-year low of 0.8%.
Discounters Aldi and Lidl held their record market shares of 4.8% and 3.6% respectively in the period, with 53% of households shopping at either outlet.
Asda was the only one of the big four chains to boost market share, with Tesco and Morrisons remaining under pressure and Sainsbury's suffering a small drop to 16.4% as its sales growth lagged behind the market at 0.3%.
The country's biggest grocer, Tesco, suffered a 4% sales fall, the largest decline among the big four. Its market share fell to 28.8% from 30.2% over 12 months.
Morrisons, which in March pledged to spend £1bn cutting prices over three years, saw its sales fall 1.9% and its market share slip to 11% from 11.3%.