Three of the most popular sun creams have been found to offer less protection than they claim, consumer organisation Which? has found.
The organisation carried out checks on 26 sunscreen lotions available from major supermarkets and retailers.
It found that three of the products did not pass all the tests, while some of the cheaper options from supermarkets Aldi and Lidl did.
The revelation comes as temperatures could soar to 30C in some parts of the UK next week, bringing an increased risk of death for the elderly and vulnerable.
The Met Office has issued a yellow health warning for all but one region of the country, which comes into effect at 8am on Monday and will remain in place until 5pm on Thursday.
Met Office chief forecaster Neil Armstrong said: “Some central and southern areas are likely to see temperatures approaching the values needed for heatwave conditions. Heatwave conditions need to remain in situ for three consecutive days, and by the middle of next week it is possible that some parts of the UK could be reaching heatwave thresholds.”
Ahead of the soaring temperatures, Which? found that products from Asda, Calypso Sun and Bondi Sands all failed some of the quality checks.
The full names of the products are Calypso Sun Press & Protect Sun Lotion SPF30; Asda Protect Moisturising Sun Lotion SPF 30 High; and Bondi Sands SPF 50+ Face Sunscreen Lotion.
Asda questioned the validity of the tests, saying independent examinations of the same batch had confirmed that its lotion provided the level of protection on the label.
Calypso Sun and Bondi Sands said all products were carefully tested and met safety standards.
But Natalie Hitchins at Which? said: “It is incredibly concerning that some big brands failed our tests and did not offer the level of sun protection claimed on the packaging.
“However, it is great to see cheaper top-quality sunscreen options available on the high street and in supermarkets at a time when shoppers need affordable options.
“Our results prove that there’s no need to splash out to keep you and your loved ones safe in the sun this summer, and that many of the supermarket own-brand products can do a good job.”
Sunscreens that passed the quality checks included Lidl Cien Sun Spray SPF 30; Aldi Lacura SP 30 Sun Lotion; Nivea Sun Protect & Moisture Lotion SP 30; Boots Soltan Protect & Moisture Spray SPF30; and Garnier Ambre Solaire Kids Water Resistant Sun Cream Spray SPF50.