Three leading sun cream brands have failed protection tests conducted by the consumer watchdog, Which?
Products manufactured by Piz Buin, Malibu and Hawaiian Tropic, which claimed a sun projection factor of 30, were all found to have an SPF of below 25.
Which? has now branded the creams "Don't buys".
The company's executive director, Richard Lloyd, told Sky News: "We found three products from three big brands that don't live up to the claims made by the manufacturer.
"Consumers will be rightly worried that products of this kind that are meant to protect you from sunburn, with thousands and thousands of cases of people now getting skin cancer … aren't living up to the very strict British standards that we tested them against.
"The manufacturers now need to reassure the public that their products are good to use and in the meantime we recommend to people that they don't buy these three products."
Which? said the results were identified in two separate British Standards tests looking into 15 products with SPF 30 - a bestseller in the UK.
The tests found that price was not linked to the level of protection provided, with more expensive products often offering a lower SPF.
The consumer watchdog said Calypso Sun Lotion SPF30 250ml, the cheapest product tested, passed both British Standard tests. It is priced at just £1.20 per 100ml.
The more expensive products were not found to offer better protectionPiz Buin Ultra Light Dry Touch Sun Fluid SPF30, which costs £11.30 per 100ml, was among those which failed.
Malibu Protective Lotion SPF30 200ml also failed tests looking into products' ability to protect consumers from a form of ultraviolet radiation called UVA, which has been liked to skin cancer and premature ageing of the skin.
EU recommendations say sun creams should over a UVA protection that is one third of their SPF value.
The Malibu cream was found to have a UVA protection factor below 10.
All three companies have rejected the tests' findings.
Malibu said: "We stand by our testing and are willing to have our formula tested again by any reputable UK testing house.
"We have nothing to hide. Our files are open for any legitimate person to come and examine."
A statement from Hawaiian Tropic said: "Hawaiian Tropic focuses fully on providing quality products and effective sun protection that meets all EU and Cosmetic Regulations.
"Consumers can fully rely on Hawaiian Tropic products, including Satin Protection SPF 30; safe in the knowledge they are protected against the sun to the level they want, need and expect."
Piz Buin said: "Our products are subject to rigorous safety and efficacy testing; in the case of sun care products, this includes SPF testing in line with the EU sun care recommendation.
"Consumers can be confident that PIZ BUIN® Ultra Light Dry Touch Sun Fluid SPF 30 provides effective sun protection at SPF 30, when used as directed."