Two of the UK's most popular brands of sunscreen lotion do not offer as much sun protection as they claim, according to the consumer organisation, Which?
It is advising consumers not to buy the products, both of which are labelled as sun protection factor (SPF) 30.
One of them is made by Boots; the other is sold under the Hawaiian Tropic brand.
However, both companies rejected the claims made by Which? and said their sunscreens were fully effective.
In the latest copy of its magazine, Which? said that both Soltan Protect and Moisturise Lotion (SP30) and Hawaiian Tropic Silk Hydration Lotion (SP30) offered only two-thirds of the skin protection they should.
The SPF figure is designed to measure the amount of protection against harmful ultraviolet B (UVB) rays from the sun - the main cause of sunburn.
Both products passed the tests for ultraviolet A (UVA) rays, which are less dangerous, but more prevalent.
The product makers have both denied the claims.
Hawaiian Tropic said all its sun tan lotions were rigorously tested, "meeting all SPF, UVA and EU requirements and regulations".
Furthermore, it said its own tests suggested that its Silk Hydration and Satin Protection lotions "exceed the label claims, even after 80 minutes in water".
Boots said it was confident that all its sun care products met the SPF labelling claims.
"Customers can rely on them to provide the level of protection expected," said a Boots spokesperson.
Eleven other sunscreens examined by Which? - using British Standard Tests - were deemed to be satisfactory.
It said own-brand products from Asda, Morrisons, Tesco and Sainsbury's all offered the SPF that was claimed.
The organisation is asking Boots and Hawaiian Tropic to improve their products so that they deliver better protection.
How do sunscreens and lotions protect us?
Philippa Roxby, Health reporter
There is a huge range of different sun products available, including lotions, mousses, sprays, gels and creams.
In general, they can use either organic or inorganic filters to absorb or reflect Ultraviolet rays.
The terms 'sun lotions' and 'sunscreens' are used interchangeably to describe many of them.
'Suntan lotion' is sometimes used to refer to substances designed to accelerate tanning with little or no sun protection factor.
Some people use the term 'sunblock' to refer to sunscreens that reflect rather than absorb UV rays.
But what's more important than the name used is the product's protection qualities, which can be found on the label.