The mini-heatwave that has hit much of the country has seen shoppers rushing to buy shorts, sun lotion, ice-cream and barbecue food, providing a shot in the arm for beleaguered retailers.

Sales of sun cream jumped almost 400% last week, purchases of burgers and other barbecue-style food were up 270% at Waitrose, and Argos sold twice as many barbecues as normal. Orders of rosé wine, which drinkers traditionally associate with hotter weather, jumped 54% at Waitrose, while Sainsbury’s said it expected a 30% increase.

As Britain’s shoppers stocked up for the change in the weather after weeks of rain, cloud and snow, the number of people out on the high street leapt 15% on Thursday, the warmest April day for almost 70 years.

“Consumers are definitely responding to the warm and sunny weather by visiting stores,” said Diane Wehrle, marketing and insights director at Springboard, which compiled the data. The balmy spell peaked in the south-east of England, where it was hotter than Spain and Italy. Temperatures reached 28.3C in St James’s Park, central London, and 27.9C in Northolt, north-west London.

On Sunday temperatures are expected to drop slightly, although forecasts of 23C in the capital would mean the London Marathon becoming the warmest on record.

The warm spell may prove short-lived, but after a chilly start to the spring contributed to a miserable time for retailers, stores said shoppers were out in force snapping up shorts, sandals and other hot-weather items. Sales of sunglasses tripled at Primark on Thursday compared with the previous week. Online retailer Asos sold 60% more pairs of shorts than this time last year and double the number of vest tops as well as 16,000 pairs of sunglasses in four days. Clarks almost doubled sales of sandals.

As temperatures rose well into the 20s across much of the UK, particularly in the south and east, Waitrose said it expected a 300% rise in sales of ice cubes and a 150% rise in beer this weekend.

“Our forecasting team closely monitor the weather as a rise in temperature of even just a few degrees will see huge shifts in buying patterns,” said a Waitrose spokesperson.

Sainsbury’s was expecting a 150% jump in sales of ice-cream and 365% in barbecue coals. Tesco expected to sell more than 8m sausages, 2m punnets of strawberries and 3m ice-lollies. Tesco BBQ meat range buyer Natalie Bastow said: “After months of rain and with temperatures set to hit 25C plus, this will be the first real opportunity Brits have had this year to get out into the garden and enjoy a barbecue with family and friends.” Argos reported a 150% year-on-year rise in sales of garden seating.

“March was tough in the UK market, but there has been a significant improvement in sales in the last week,” said Mike Shearwood, chief executive of footwear chain Clarks.

Other fashion industry bosses agreed the increase in sales had been dramatic in some parts of the country. One said that sales tripled at one outlet on Thursday as the sunshine hit.

But he said that the burst of activity was likely to be short-lived and would not help compensate for a difficult March. A survey of Visa card users indicated that consumer spending slid 2% last month, while the number of shoppers was down 6% compared with March last year, according to Springboard, after strong winds and snow hit the UK.

The Easter bank holiday weekend, usually the start to the gardening year, was a washout as heavy rain in many areas kept people indoors. “It’s slightly better, but everybody will be in the park or the pub. They will not be flooding to the shops,” the fashion boss said.