Two sun creams from leading brands are failing to provide the protection they claim, according to tests by a consumer watchdog.
Boots Soltan Protect & Moisturise Lotion SPF30 (200ml) and Hawaiian Tropic Silk Hydration Lotion SPF30 (180ml) both offered only around two-thirds of their claimed SPF (sun protection factor), Which? said.
The consumer group said the Silk Hydration Lotion was "worryingly" the second Hawaiian Tropic product to fail its tests in the last two years.
It has branded the creams as "don't buys" in its investigation, which used British Standard tests to check 13 products with an SPF of 30 - a bestseller in the UK - on their SPF and UVA protection.
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Piz Buin Ultra Light Dry Touch Sun Fluid SPF30 (150ml) and Hawaiian Tropic Satin Protection Sun Lotion SPF30 (200ml) both failed the tests last year, and Which? said it continued to recommend that consumers avoid these products.
SPF indicates the amount of protection sunscreens offer against UVB, which is the main cause of sunburn.
Which? said effective products did not have to be expensive, finding that "affordable" own-brand lotions from Asda, Morrisons, Tesco and Sainsbury's all offered the SPF claimed.
Which? executive director Richard Lloyd said: "We're very concerned that a further two popular sunscreens have now failed our tests.
"Consumers must be able to trust and rely on the information provided by manufacturers so it's disappointing to see well-known brands falling short.
"We want them to take action to ensure their products deliver the promised protection."
Cancer Research UK health information manager Dr Claire Knight said: "In the UK more than eight in 10 skin cancers could be prevented by enjoying the sun safely and not getting sunburnt.
"The SPF and star rating are the two most important things to look out for when choosing sunscreen so, whichever brand you choose, it's vital you can trust the information given on the bottle.
"Using sunscreen isn't the only way to protect your skin from strong sun. Covering up with a shirt, hat and sunglasses and spending time in the shade are the best ways to reduce the risk of sunburn and skin cancer."
Hawaiian Tropic said all its products, including Silk Hydration Lotion SPF 30 (180ml) and Satin Protection Sun Lotion SPF 30 (200ml), were rigorously tested, meeting all SPF, UVA and EU requirements and regulations.
It said: "Hawaiian Tropic invests considerable resources and research in product development and testing to ensure consumer safety, and guarantees the SPF and UVA claim on all products.
"Consumers can fully rely on Hawaiian Tropic products, safe in the knowledge they are protected against the sun to the level they want, need and expect."
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