Food sales were up on a year ago as the purchase of home-baking products, potatoes and stewing meats were replaced with salads, ice-cream and barbecue foods as the weather improved.
Non-alcoholic beverages, wines, beers and spirits also sold very well compared with a much cooler May last year.
Despite the apparent arrival of summer lifting shoppers' spirits, the underlying trend of customers searching for good value continued, the BRC added.
Online, mail-order and phone sales of non-food items also showed stronger growth, up 12.4% against growth of 10.4% last year.
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