Four leading "once-a-day" sunscreens are not living up to their claims, a watchdog has warned.
Which? found that the protection offered by Soltan Once Invisible 8hr Sun Protection SPF30 (200ml), Piz Buin 1 Day Long Lotion SPF30 (150ml), Riemann P20 Once a Day Sun Protection SPF30 (200ml) and UltraSun Family SPF30 (100ml) decreased by an average of 74% after six to eight hours.
This meant that over the course of a day an SPF30 "once-a-day" sunscreen could drop to offer as little protection as SPF8, the consumer group said.
Which? said it used British Standard testing on the four products as well as a second test in which the sunscreens were applied to the backs of volunteers who spent a day in a laboratory before the SPF was tested again after six to eight hours depending on each brand's claim.
The watchdog noted that similar once-a-day claims are not permitted in Australia, where anything that leads consumers to believe sunscreens do not need to be regularly reapplied is forbidden.
Which? director of policy and campaigns Alex Neill said: "Our testing shows that these sunscreens just don't live up to their 'once-a-day' claims so people should reapply sunscreens regularly to ensure they have protection from the sun.
"With more than 100,000 people diagnosed with skin cancer in the UK each year, some manufacturers need to do more to ensure their sunscreens live up to the claims on the packaging."
The British Association of Dermatologists said such extended-wear products should not necessarily be avoided but should be used similarly to other sunscreens and reapplied.
Cancer Research UK said: "The amount of protection you get depends on how well you put it on. It's easy to miss bits when you're applying sunscreen.
"Cancer Research UK recommends you reapply regularly to help get even coverage of your skin."
Which? also tested 11 widely available regular sunscreens to see if they offered the SPF30 they claimed, with Hawaiian Tropic Satin Protection Ultra Radiance Lotion (180ml) failing twice after it was found to provide "significantly less" protection than it claimed.
The study found that even the cheapest sunscreens can provide good protection, with own-brand products from Asda, Lidl and Wilko all passing testing.
The cheapest sunscreen that passed the SPF test was the £2.79 Aldi Lacura Suncare Moisturising Sun Spray SPF30 200ml.
Dr Chris Flower, director-general of the Cosmetic, Toiletry & Perfumery Association, added: "Which? has set up its test for extended-wear products very strangely. Manufacturers test products in circumstances related to normal or expected use.
"We normally apply sunscreen to areas of skin that will be exposed to the sun but we don't apply sunscreen to areas that will remain covered under clothes. Exposed areas will not be subject to rubbing against a T-shirt for six to eight hours, so we feel this is unrealistic as a test and believe the criticisms from Which? are not justified.
"Extended-wear sunscreens are valuable for people with an active outdoor lifestyle for whom re-application of sunscreen is difficult or impossible. Careful formulation and extensive testing by the manufacturers ensure that we can be confident of the SPF labelling for both extended-wear and traditional sunscreens."