Anyone buying sunscreen this summer needs to know about this 'hidden' three-letter sunscreen code that will save you money, according to Martin Lewis.

The financial expert has advised anyone buying suncream to turn the bottle over and take a look at the three-letter code on the back.

In a new post, he explains how it works: "As we're racing towards summer, just a quick tip on sunscreen, when you get your sunscreen, turn the bottle around, because on the back, you'll see a PAO number, usually 12 or 24 months.

He added: "PAO stands for period after opening, and it's how long the sunscreen is still going to give you protection for once you've opened it.

"So then, when you do the what they write that onto the sunscreen and you know if you're trying to use it in a year's time, whether it's still valid or not and not just think that it's too old and you should chuck it. Save yourself a little bit of cash."

What will happen if you use expired sunscreen?

The skincare experts at Kiehl’s warn that the sun protection elements won’t be as effective after the product expires.

"Active ingredients break down over time and there’s potential for chemicals to alter," they say.

"Sunscreen should never be used after its expiration date as it simply won’t provide the protection advised and could leave you in danger of sun damage. Expired products will also become full of bacteria and transferring that to your skin might cause redness, a mild rash or an eye infection.

"Because of this, expired products need to be discarded. If you have sensitive skin, then you’ll want to be extra vigilant."

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Where should you keep sunscreen?

Sadly, not in the bathroom, as the humidity and changes in temperature can cause issues for most formulas.

Keep them away from spots where they’ll get too hot like windowsills or above radiators and try to avoid exposing them to too much light. Instead, keep your products in a cupboard or drawer and don’t forget to reseal them after using.

If you’ll be putting your fingers in a jar, always make sure they are clean too. Just remember, if it doesn’t smell right or looks weird, it’s time to throw it out.