A basket of everyday goods, including milk, bread and vegetables, now costs 0.4% less than it did this time last year.
The latest figures from market analyst Kantar Worldpanel mark the first fall recorded since it began monitoring the market in 1994.
Total grocery sales fell by 0.2% in the 12 weeks to 9 November, compared with the same period last year, and price deflation is forecast by the firm to to continue well into 2015.
The latest figures show Aldi continuing to benefit, with sales 25.5% higher than last year and a record-high market share of 4.9%. Lidl has also performed strongly, with a 16.8% sales increase bringing its market share to 3.5%. Waitrose is up 5.6% to a 5.1% share.
Among the big four, Asda has recorded the best performance, with sales dropping in line with the overall market and share holdidng steady at 17.2%. Sainsbury’s and Morrisons both saw their share fall, down 2.5% and 3.3%, respectively. Tesco’s sales are down by 3.7%.