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.

Professor Brian Diffey adds that sun creams generally offer ‘very good’ water resistance these days (though be aware water resistance claims also come with caveats, and the products we tested didn't hold up well to real-life conditions in our 2016 investigation), which is good if you're planning on spending time in the pool or in the sea. But over the course of the day swimming will wash away some of the protection. Professor Brian Diffey, of BAD, says: 'We recommend against relying on any sunscreen product for long periods without reapplication, whether advertised as an extended wear product, or not. 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.The regular reapplication advocated by most sunscreens protects us against this,’ he says. To apply sun cream properly you should: 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. Mineral sunscreens fail Which? SPF and UVA tests - find out what happened when we put pricey mineral formulations to the test in our independent lab

Apply a generous amount of sun cream - around one teaspoon per body part - at least 15 minutes before you go into the sun so it soaks in. The British Association of Dermatologists (BAD) says frequent reapplication of sun cream is important, regardless of how long any product claims to give protection for. Reapply sunscreen after swimming, sweating heavily, towel drying, or rubbing your skin in a way that might cause the cream to rub off. 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. We made multiple attempts to get in touch with Calypso, but the brand did not respond to any of our questions or send us a comment. RiemannProviding silky smooth protection from harmful UVA and UVB rays, Piz Buin 1 Day Long 200ml SPF 15 is just the sunscreen you need to throw in your suitcase this summer. Combining a gorgeous scent with Vitamin E for a luxurious pamper session in a bottle, it feels oh-so-sumptuous on the skin. Don't just rely on sunscreen either: use clothing, hats, sunglasses to cover up, and seek shade, particularly when the sun is strongest, between 11am and 3pm - regardless of whether you are wearing sun cream. 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. What's more, even if you follow all the rules, it's possible you won't get as much as protection as you expect.

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). A spokesperson said: 'Soltan Once products clearly state how they should be used and when to reapply. We are confident in the claims for the Soltan Once range and refute the suggestion that they are misleading. 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. Over the course of a day, he added, 'swimming, drying, sweating, getting dressed and undressed' can cause sun cream to be wiped or washed away.Nobody applies sunscreen perfectly uniformly to all exposed skin and it is inevitable that we will miss patches of skin.

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.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. Casting incorrect assertions in this manner is misleading and confusing to customers and we are disappointed that Which? has chosen to do this.' Calypso So, if you are only applying your sunscreen once a day, you are at risk of spending hours in the sun with little or no protection to those areas of skin that may have been missed.'