The bank holiday weather forecast is looking positively tropical, meaning it’s time to fish out that sun cream. But if you’re planning on taking a swim, experts have warned that you might not be getting the protection you expect from your SPF.

Consumer watchdog Which? tested water resistance claims made by two popular sunscreens – one own-brand and one well-known international product – and found the sun protection factor (SPF) dropped by up to 59% after forty minutes in salt water.

Water resistance claims are made on the majority of sun protection products, yet these findings “expose serious flaws in the current testing regime”, Which? said.

The current testing process involves one volunteer to be immersed in a bath of tap water, which circulates to simulate “moderate activity”. Manufacturers can claim a product is water resistant if the SPF drops by up to 50% after two 20-minute periods of immersion. 

Other countries including Australia and the United States have stricter requirements where the SPF on the label must be the SPF it provides after immersion in tap water for designated periods of time.

Which? carried out more rigorous tests in salt water, chlorinated water and fast-moving water – conditions that more closely resemble those encountered on holiday – and found that the well-known international product’s SPF dropped by 59% after 40 minutes of immersion in salt water and in moving water.

The popular own-branded product’s SPF dropped by 34% in both salt water and chlorinated water. But in reality, sun protection is likely to drop even further, as factors like reflection (from water), heat, light, sweat, towelling and rubbing all reduce the protection of sunscreens. 

Which? believes the current requirements around the water-resistance claim are unrealistic to the point of being meaningless and said consumers could be relying on a product to provide a level of protection that it is incapable of delivering, “putting them at risk in the sun”.

They added there’s no way for consumers to know what SPF they’ll end up with after going into the sea or pool when they make a purchase.

In separate tests, Which? looked to see whether 15 widely available sunscreens met their SPF claims, and all passed. 

Nikki Stopford, director of Research and Publishing, said: “Our research shows water resistant sunscreens don’t live up to their claims when subjected to rigorous tests – raising serious questions about the current guidelines. 

“With 15,400 new cases of melanoma each year, manufacturers should be required to robustly test their products and make only claims that can be relied on, ensuring holidaymakers know they can trust their sunscreen to protect them.”