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. 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. There’s a long list of ingredients that are used as chemical absorbers, so it’s easiest to simply look for products that are labelled ‘sensitive’ rather than search for specific ingredients. SPF make-up: does it really offer protection from the sun?

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. It takes longer for those with darker skin that rarely burns – it can take 25-40 minutes for the darkest skin type. 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. 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.

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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. 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. Consider the time of day – If you're spending time in the sun without wearing sun cream, you should only do so before 11am or after 3pm, as the middle of the day is when you're at the greatest risk of getting burnt. This tends to mean that two commonly used chemical UV filters linked to coral bleaching – oxybenzone and octinoxate – aren’t used.

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. 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.Physical/mineral products reflect visible light as well as UV, which can make the formulations opaque and leave a white residue on the skin. Some formulations use nanoparticles of titanium dioxide and zinc oxide – the minerals responsible for the residue. Nanoparticles are smaller, and as they scatter visible light – rather than reflect it – the sun creams are transparent. How to use sun cream

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”.

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When choosing sun cream for your child, look for a short ingredients list and the terms 'fragrance free' and 'hypoallergenic'. The WHO recommends 35ml of sun cream (seven teaspoons' worth) to cover an adult, so use around a quarter to a half of this depending on the size of your child.