Using so-called once-a-day sunscreen is not fully effective against the sun's harmful rays, research by Which? has found.
It tested four brands which claim that users only need to apply them once during the day.
It found that after six to eight hours the average Sun Protection Factor (SPF) offered decreased by 74%.
However the claim was rejected by some of the manufacturers, who said their own testing had different results.
According to the Which? study, a sunscreen offering an SPF 30 could actually drop to an SPF 8 over the course of a day.
Claims about "once-a-day" sunscreens are not allowed in Australia, where there are strict regulations about such assertions.
Anything that leads consumers to believe sunscreens do not need to be frequently re-applied is forbidden.
Which? believes similar rules should be introduced in the UK.
The brands tested by Which? were:
- Soltan Once Invisible 8hr Sun Protection (SPF 30) 200ml
- Piz Buin 1 Day Long Lotion (SPF 30) 150ml
- Riemann P20 Once a Day Sun Protection (SPF 30) 200ml
- Ultrasun Family (SPF 30) 100ml
Boots - the maker of Soltan - said customers could be confident of getting the expected level of protection over the eight hours.
But it admitted that protection levels do go down over time.
"It is expected that the level of SPF in any 'once a day' sun protection will reduce throughout the day," a Boots spokesperson said.
"This is why we formulate Boots Soltan Once 8 Hour Sun Protection SPF30 to a higher level of SPF so that our customers can be confident it will not reduce below SPF 30, and they will get the right level of protection for a full eight hours."
Another of the brands concerned, UltraSun, said that Which? had averaged out all the results, and had not provided them with their individual scores.
"Our own tests show phenomenal results," said Abi Cleave, the managing director of Ultrasun UK.
"We are really responsible about our claims."
Piz Buin said its 1 Day Long products carry clear directions on the need for re-application.
It also said that once-a-day sunscreens should continue to be available in the UK.
The industry body, the Cosmetic, Toiletry and Perfumery Association, was critical of the way Which? carried out its tests, using volunteers wearing t-shirts.
It said that in normal conditions, sunscreen would not be subject to rubbing, as it would have been in the tests.
The British Association of Dermatologists advised that extended-wear sunscreens should be used in the same way as conventional products - in other words they should be regularly re-applied.
Last year Which? said some sunscreen brands did not offer the protection factors they claimed.