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.

From April to September, when the UV Index is generally higher. It takes 10 to 15 minutes of sun exposure each day for those with fair-to-olive skin to get enough vitamin D. 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.

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. 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. 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. 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. 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.'

This tends to mean that two commonly used chemical UV filters linked to coral bleaching – oxybenzone and octinoxate – aren’t used. The industry guideline for water-resistant sun creams allows the SPF of a product to drop by 50% after a total of 40 minutes in water. Water-resistance testing also takes place in what is essentially a bath – using tap water. 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. 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 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.Mineral sunscreens in particular often promote their environmental credentials, the most common being that they’re ‘reef-safe’ or 'ocean-friendly'. 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 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? People can still use these types of sunscreens, but you should treat them like any other sunscreen, and reapply regularly,' he said.

For any product to provide the SPF it claims, you need to apply 2mg per square cm – which means around a teaspoonful of product for your face. And, just as with sun cream, it needs to be reapplied regularly. What's more, even if you follow all the rules, it's possible you won't get as much as protection as you expect.

It's common for sun creams to claim to be 'water resistant', but this doesn't mean that they're waterproof. 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. 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.

Reapply sunscreen after swimming, sweating heavily, towel drying, or rubbing your skin in a way that might cause the cream to rub off. Casting incorrect assertions in this manner is misleading and confusing to customers and we are disappointed that Which? has chosen to do this.' Calypso 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. Riemann Nobody applies sunscreen perfectly uniformly to all exposed skin and it is inevitable that we will miss patches of skin.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. Remember, sun cream is just one part of staying safe in the sun and it's also really important to encourage kids (and adults) to: 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. It takes longer for those with darker skin that rarely burns – it can take 25-40 minutes for the darkest skin type. 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.