The official advice given to sun-bathers to use factor 15 sunscreen is insufficient, a top doctor has warned.

If people wish to prevent sunburn and the risk of skin cancer, they should slap on factor 30, said Dr Ike Iheanacho, editor of the influential Drug and Therapeutics Bulletin (DTB).

The National Institute for Health and Clinical Excellence (Nice) published guidance in January saying people should wear broad spectrum (UVA and UVB) sunscreens with a minimum factor 15 sun protection (SPF).

However, this level of SPF is too low and not based on how people actually use sun creams, says an editorial in the DTB.

‘In reality, people using sunscreens typically apply much less than this and get no more than half, at best, of the protection indicated by the labelled SPF,’ says Dr Iheanacho’s editorial.

It says sunscreen runs off the skin, which means it has to be frequently re-applied.

‘Using it “adequately” is also costly, since whole-body coverage for a single application for an adult at 2mg/cm2 requires around 35ml of sunscreen,’ the editorial adds.

‘Applying this much at least every two hours, as Nice also recommends, would require a standard 200ml bottle of sunscreen every two to three days.

Dr Iheanacho said: ‘In DTB’s view, Nice’s recommendation to use sunscreens with an SPF as low as 15 is a blunder that overlooks the key evidence. This advice needs urgent review and correction.’

The DTB advises that people with fair skin can safely expose their hands, arms, face or back to sunlight in the UK from April to September, for 15 minutes two or three times a week.

More than two Britons under 35 are diagnosed with the deadliest form of skin cancer every day.

Rates of malignant melanoma have tripled among those aged 15 to 34 since the late 1970s, according to Cancer Research UK.

Then, there were 1.8 cases of melanoma per 100,000 people in this age group, rising to 5.9 now.

Professor Mike Kelly, director of the centre for public health excellence at NICE, said: ‘We felt it was important, in producing this guidance, to maintain a balance – recognising on the one hand the very real dangers of skin cancer, but also remembering on the other hand that we should not extrapolate from research carried out in much hotter, sunnier climates than our own.’

He said NICE commissioned an expert report on the use of suncreen to prevent skin cancer in England.

Professor Kelly added: ‘According to this evidence, SPF 15 should be sufficient as long as it is applied adequately.

‘However, the guidance also acknowledges that to take account of people not applying sufficient quantities of sunscreen, SPF 30 was also recommended in an expert paper.’