Barclaycard said warm weather helped spending in pubs and restaurants, as well as garden centres and clothes stores.
BRC chief executive Helen Dickinson said: "Today's figures bring better news for the UK's retailers. After two months of flat sales May saw growth of 1.4 per cent. While still not spectacular, it is in line with the 12-month average of 1.5 per cent.
"Clothing made a big comeback this month after suffering declines in April. This appears to be due to consumers waiting for just the right moment before embarking on their pre-summer spending. However, with signs that the UK’s economy is slowing it’s unlikely that this is the beginning of a complete reversal of fortunes. The uncertain outlook means that customers will remain cautious with their spending, therefore we expect sales figures to remain volatile for the time being.
"While this month's pick-up in sales will come as a welcome relief, it's still a challenging time for retailers. Shop price deflation continues and intensity of competition is fierce. Controlling costs and improving efficiency and productivity will be crucial for retailers as they continue to navigate increasingly tough trading conditions."
KPMG retail head David McCorquodale said: "Overall, retail sales showed some improvement in the month, up 1.4 per cent in total versus May last year. Despite rain dampening the May Day mood, fashion sales finally lifted following a tough three months of trading for retailers. The appearance of some spring sunshine encouraged consumers to hit the sales and take advantage of early summer promotions.
"The standout category, however, was children’s footwear, and children’s clothing also saw a boost as parents kitted out the kids for the warmer weather and half term holidays. Mild relief also continued for the Grocers with total food and drink sales remaining positive in the month.
"With a summer of sport kicking off in June and festival season getting underway, retailers will be hoping that the feel good factor coupled with a dose of summer sunshine will keep the tills ringing over the summer months."