Sunscreen lotions may not protect against skin cancer, according to a study by British doctors.
The doctors tested three popular brands of sun lotion
They have found some leading brands fail to stop the sun's damaging rays from penetrating the skin.
Writing in the Journal of Investigative Dermatology, they said further research is needed to see what, if any, creams are effective.
The doctors said staying out of the sun or covering up when outside is the best way to protect against skin cancer.
Professor Roy Sanders and colleagues at the research charity Raft carried out tests on samples of skin.
This skin had been removed from patients with their consent, following a breast reduction operation, for example.
The doctors exposed the skin to UVA light at intensities similar to that of sunlight.
Exposure to UVA rays is known to cause premature ageing of the skin and increase the chances of skin cancer.
They penetrate the skin causing the release of free radicals, which can cause damage to DNA, which can in turn cause cancer.
Sunscreen lotions are supposed to stop this from happening.
The doctors applied three commonly available high factor sunscreen lotions to their skin samples at the recommended doses.
Their tests showed that while the lotions prevented the sun from burning the skin, they did not stop them from penetrating the skin.
"They don't seem to offer protection against free radicals," Professor Sanders told BBC News Online.
Last line of defence
"This is a problem because if people are using these creams on the supposition that they do offer protection then they may be putting themselves at higher risk of skin cancer.
"If you use a sunscreen which protects against UVA and take more sun as a result of that, you may be effectively increasing the dose of the UVA and therefore greatly increasing the chances of getting a malignant melanoma.
Professor Sanders said people in Australia were adopting the right approach by encouraging children to cover up with clothing rather than relying on sunscreens when in the sun.
"My advice to people is to keep out of the sun or cover up," he said.
Dr Mark Birch-Machin, a skin cancer expert at Cancer Research UK, said the findings backed up other studies.
He told BBC News Online: "The message from this study is that sunscreens do not provide total protection against skin cancer. They are just part of a toolkit.
"They are almost like a last line of defence. People should stick on a hat, a t-shirt and stay in the shade. We shouldn't rely exclusively on sunscreens."
The high street chemist Boots, which manufacturers its own range of sunscreen lotions, said people should be sensible when out in the sun.
"You shouldn't assume that just because you're wearing sunscreen you're completely protected," said a spokeswoman.
"However, wearing sunscreen and being sensible in the sun is far better than the alternatives."
Read a selection of your comments on this issue below.
As a very pale person living in a sunny climate, I have learned that sunblock, though imperfect, is better than nothing. After being diagnosed with melanoma this summer (at 25), I did some research into skin cancers, and their incidence are on the rise, even among my age group. We need to be sure we don't just blame sunblocks for not working well enough, or the sun for shining, but also look at the other environmental factors like air pollution and greenhouse gasses that make the sun more dangerous now than it ever has been.
Wearing sunscreen with a protection of 15 or higher, as well as, monitoring the time spent in the sun can help prevent skin cancer. Those who have outside commitments, such as a job, should take all precautions: wear a hat, sufficient clothing to cover the body, as well as sunblock.
Sarah Drew, USA
I have doubted the safety of sun creams for some time. They seem full of petrochemical derivatives which the body absorbs through the skin. They are cleverly packaged, but are they just full of chemical waste? Generally speaking I think it is much easier for people to pay £8-£10 and slap on some sun cream than to address a need for a change in behaviour.
Please, can we stop the hysteria? Is it not logical that pale people, unlikely to ever go anything from white to pink and back again, should not be in the sun? Can we concentrate more on trying to stop ozone damage and foster education, rather than consigning us all to a pale, wan future? Besides, the sunscreen companies are going to go mad over this one.
I think it is dangerous to imply that wearing sunscreen may have no benefit. It has taken many years for the message to filter through that we should look after our skin and protect in sunlight for medical reasons, not just to avoid premature ageing. Wearing sunscreen can surely not be a mistake even if the protective effect MAY be less that originally thought. Some cream is still better than no cream (and clearly DOES help to slow premature ageing.
Sunlight is healthy if done with moderation. It is a vital factor in Vitamin D synthesis and improvement of depression as those living in northern latitudes are well aware. The study doesn't seem to address the tanning salon variable. With their numbers on the rise they seem to represent a higher risk factor by making UV light available year round.
I have suffered from a melanoma on the instep of my foot. How can the sun have caused this? My consultant said that melanomas can occur in the mouth, inside the ear etc. Do we really know for sure that the sun does indeed cause melanomas, and not just basal cell carcinomas which are not so deadly.
I am no doctor, but I think this advice makes a lot of sense. In my heart of hearts, I have always suspected that most of the sunscreen lotions do not offer any real protection. In fact, a lot of them are simply marketing gimmicks. I think the advice given in this article is very sensible and people should follow these, rather than depending on lotions that claim to block UV rays.
Pratick Mukherjee, GA, USA
I never use sun lotions, either at home or when I'm travelling. I'm afraid of what chemicals the companies put in their products regardless of their claims. When in strong sunshine, I always wear a hat and protective clothing. Now that I've read your report, I'm even less likely to use sun screens.
Paul Borger, Denmark
I have always thought this to be true. Just rubbing some magic chemical on your skin is not going to prevent the power of the suns rays.
I have always blocked the sun with cloths, as I think most sun-tan-lotions CAUSE cancer themselves.
Rory Lambert, UK
I have always wondered why Southern Europeans have considerably longer life expectancy than the English and Scottish. Shouldn't they be dying like flies from skin cancer? Obviously frying yourself for hours when you are not used to it is stupid, but the whole skin cancer scare seems wrong to me somehow.
Peter Schofield, England