Which? has criticised once-a-day sun creams after its research found that these products decline in SPF protection over the course of a day.

Researchers tested several one-use sunscreens by applying them to the backs of volunteers who then wore a t-shirt and sat on a chair. They discovered that the products declined in SPF protection by an average of 74 per cent after six to eight hours, so that a cream offering SPF30 when fi rst applied only provided SPF8 later on.

Commenting on the findings, the Royal Pharmaceutical Society’s chief scientist Professor Jayne Lawrence, said: “The effect of sunscreen also varies from person to person depending on skin type, strength of the sun and if you use enough sunscreen in the first place.”