We award Which? Best Buy status to products that we've independently reviewed that meet all our tough criteria: they must pass independent lab tests for UVA and UVB sun protection, and be easy and pleasant to apply. We saw an average 74% decrease in SPF protection at the end of the tests. We shared our concerns with Cancer Research UK (CRUK) and the British Association of Dermatologists (BAD). Both advise against relying on any sun cream for extended periods in the sun. For more details on the importance of vitamin D and where to buy supplements, see our in-depth vitamin D guide . Other ways to stay safe in the sun You can buy sun cream in pharmacies, supermarkets and discount stores, with some of our tested options coming from Asda, Boots, Garnier, Nivea and Wilko.

Find out what you need to know about 'once a day' sun creams, water-resistant sun creams, sensitive sun screen and the best kids' sun cream too. Physical or mineral sun creams create a barrier on the skin that acts like a mirror, reflecting and scattering the UV light. Avoid sun exposure between 11am - 3pm. Seek shade and always protect children with a hat, loose clothing and high SPF protection. Babies less than six months old should be kept out of direct sunlight, according to the NHS, and the skin of young children should be protected from the sun when UV levels are high. Calypso Once A Day may cause staining on certain fabrics and surfaces. Always cover seats and cushions. May discolour nail products.We tested two popular sun creams in chlorinated water (to mimic a swimming pool) and salty water (to emulate the sea). We also added another scenario using fast-moving tap water to see what effect this had. Most of the time, the products weren't as effective in our more realistic conditions as they were in tap water.

It takes longer for those with darker skin that rarely burns – it can take 25-40 minutes for the darkest skin type. Some people worry that using sun cream might prevent them from getting enough vitamin D, but this isn’t the case unless you really overdo it. This tends to mean that two commonly used chemical UV filters linked to coral bleaching – oxybenzone and octinoxate – aren’t used.

We asked Boots a series of questions about its Once sun cream range and asked it to respond to our concerns about 'once a day' claims.

They were allowed to wear t-shirts and sit on a chair (we did this because if you lie on a sun lounger or towel, get sandy, or wear a t-shirt on holiday it’s possible your single-use sunscreen could be rubbed off over time). The regular reapplication advocated by most sunscreens protects us against this,’ he says. To apply sun cream properly you should: Yes, you can - but you’ll also need to reapply frequently, just like you would if you were on the beach staying dry, and immediately after towel drying.This is because, she said, the length of protection can vary depending on a person's activities - including how much they sweat and whether they towel dry, for example. It's clear that there are plenty of conditions to fulfil to get the claimed protection from once-a-day products, which may be hard to achieve if you're on a day out in the sun. We agree that the claim “Once a day” can be anticipated in a wrong way by the consumer, and it is one of the reasons why we changed our durability communication from “Once a day” to “Up to 10 hours”. A spokesperson told us that all of its sun creams are designed for 'longevity' but that it found the use of the word 'durable' more 'appropriate' than once a day. Eat well, live better and stay healthy - sign up for our free monthly Food & Health newsletter for the latest insights delivered straight to your inbox

In 2016 Which? put several once-a-day sunscreens to the test, investigating how long they lasted on volunteers in a controlled lab experiment, and found they didn't offer nearly as much protection as claimed. Even with our test subjects just sitting on a chair in a T-shirt, we saw substantial reductions in the level of sun protection, so we don't think you can rely on once-a-day sun cream to keep you protected all day. Since our 2016 investigation, the CTPA has advised brands to voluntarily stop making 'once a day' claims and move towards using the word 'durable' instead. Apply evenly and generously, smooth in gently. We recommend a minimum quantity of 35ml for all over body protection.

A spokesperson added: 'As with all sunscreens, it is important to follow the instructions for application and use; and sunscreen should not be used to intentionally stay longer in the sun. Sunscreen is just one part of a sun safe regime.' Boots People can still use these types of sunscreens, but you should treat them like any other sunscreen, and reapply regularly,' he said. Cover up – wear sunglasses to protect your eyes and grab a hat if you need to keep your face out of the sun. You might also want to wear a T-shirt, which can help to cover up typically over-exposed areas such as shoulders.