Two leading sun creams are failing to provide the UV protection they claim, according to consumer watchdog Which?

Boots Soltan Protect & Moisturise Lotion SPF30 and Hawaiian Tropic Silk Hydration Lotion SPF30 both offered only about two-thirds of their claimed SPF, Which? said.

It has branded both the creams as “don’t buys” in an investigation which tested 13 products with an SPF of 30.

The consumer group said the Silk Hydration Lotion was, “worryingly”, the second Hawaiian Tropic product to fail its tests in the past two years, as well as being “more than four times as expensive as some of our Best Buys”.

The two other products it had failed last year were being reformulated, but one of 2014’s failed brands, Piz Buin Ultra Light Dry Touch, will still be available in its old formula until 2017, a product Which? advised customers to still avoid.

Sun creams are designed to protect skin from UVA and UVB radiation, which may increase the risk of developing skin cancer. SPF indicates the amount of protection sunscreens offer against UVB, which is the main cause of sunburn.

The tests conducted by a Which? sun protection expert took place on humans, with a precise amount of sunscreen added to a consistently-sized area on 10 volunteers’ backs. They had to lie still and a lamp was shone on their back, simulating the sun’s rays.

The day after each exposure, their backs were checked and researchers assessed the smallest dose of the UV light required to cause redness, with and without sunscreen.

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 it was vital that customers be able trust what the sun cream said on the bottle. “In the UK more than eight in 10 skin cancers could be prevented by enjoying the sun safely and not getting sunburnt,” she said.

The British Association of Dermatologists said customers relied on the SPF rating when choosing sun cream. “There are various types of sunscreen that use different means of protection, however. With all of these different approaches the SPF is what is used to determine their protective qualities,” said spokesman Matthew Gass.

Hawaiian Tropic said all its products were rigorously tested, meeting all SPF, UVA and EU requirements and regulations. “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,” a spokesman said.

“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.”

A Boots UK spokesman said: “Customer safety is paramount and we rigorously and independently test our products to ensure the appropriate level of safety and efficacy in compliance with EU Regulations.

“We are confident that all of our sun care products, including Boots Soltan Protect & Moisturise Lotion SPF30, meet the SPF labelling claim and customers can rely on them to provide the level of protection expected.”

“We believe that it’s better to use sunscreen that will offer some protection rather than none at all,” said Lisa Bickerstaffe of the British Skin Foundation, adding that customers could also “look for a four- or ideally five-star UVA rating on the bottle which will help protect from UVA radiation, associated with skin ageing.”