Which? tested how much energy it took to redden the backs of volunteers exposed to rays from ultraviolet (UV) lamps, with and without sunscreen, and applied the lotions to glass plates to measure the amount of light they filtered.

It carried out some tests in 2023 and some in 2024.

An Asda spokesperson told BBC News: “We recently had ourAsda Protect Moisturising Sun Lotion SPF 30 High retested by a leading external provider using internationally recognised testing methods.

“These test results confirmed that this product has a sun-protection factor of 31.5 – and therefore, we do not recognise the test results Which? has published.

“When Which? reviewed this same product in 2022, they awarded it ‘great value’ at that time and it passed their SPF test.

“The product formulation has not changed since then, so we would question the validity of their test.”

A spokesperson from Calypso Sun told BBC News: “We would like to thank Which? for sharing their 2024 study results – however, all our products are carefully and independently tested and have passed, in accordance with EU standards and regulations.

“Calypso Sun will continue its proud commitment in providing safe-sun protection products for all the family, a promise we have consistently kept over several decades.”