Non-food shop prices fell by 2.1% in December compared with the same period last year.
The British Retail Consortium-Nielsen Shop Price Index reported that the decline in non-food shop prices from 1.1% in November to 2.1% in December was the fastest decline since January 2017.
Overall shop prices for the month fell by 0.6% on last year, while food inflation rose by 1.8%.
Shop price deflation hit a four-year low in November.
Helen Dickinson, chief executive of the British Retail Consortium, said: “After several months of shop prices teetering on the edge of inflation, December saw them retreat deeper into deflationary territory.
“Prices in December fell at the fastest rate since March this year, when only last month we saw the shallowest rate of deflation for four years.”