Growth in non-food products was up by 13.7 per cent in May year-on-year, a figure higher than both the three-month and 12-month averages of 9.9 per cent and 12.2 per cent respectively.
In the three months to May, store sales decreased by 1.6 per cent - the steepest drop since December 2012, according to the BRC-KPMG online retail sales monitor.
Online sales accounted for 21.2 per cent of the total non-food sales across the UK, compared to 18.9 per cent in May last year. This is the fifth month in a row that the online penetration of non-food sales was above 20 per cent.
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Helen Dickinson, chief executive of the British Retail Consortium (BRC), said: "Retailers saw positive growth in all categories while fashion sales throughout the month proved particularly popular with online shoppers.
"As contribution from stores was negative, online sales took more than their proportionate share of overall non-food growth. The online penetration rate reached the third highest ever recorded, after Black Friday in November and the January sales.
"Retailers are confidently using their online channels...This is evidently where businesses need to direct investment to ensure they remain competitive and meet the demands of the increasingly digital savvy consumer."