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Almost one in every 20 children has used a sunbed - despite under-18s being banned from using the machines, a new poll suggests.
Since 2011 it has been against the law for children to use sunbeds in England and Wales, and in Scotland since 2009, but a new survey found that 4.5% of under-18s are still using the machines.
Wales and Scotland also included measures for all sunbeds to be supervised by trained staff.
But Cancer Research UK said that many children have "side-stepped'' the rules by tanning at home.
And youngsters in England are using unstaffed, coin-operated sunbeds in salons, a spokesman said.
But the number of youngsters who use the machines has reduced since the law came into force - from 6.8% in 2008/9 to 4.5% now, according to research which will be presented at Public Health England's (PHE) annual National Cancer Intelligence Network conference in Birmingham.
The survey, conducted by PHE and the University of the West of England, also found that half of those who had ever used a sunbed had burned at least once as a result.
And every single child who admitted using an unsupervised coin-operated bed on a regular basis said they had burned at least once.
The poll, funded by Cancer Research UK, was conducted on 3,000 11 to 17-year-olds from across England, Wales and Scotland.
It also found that four in ten of the youngsters said they never been given health advice on the potential harms of sunbeds, or differences between skin types.
Professor Julia Verne, lead author and strategic public health lead of PHE's National Cancer Intelligence Network, said: "The number of children using sunbeds in Great Britain has fallen since the under-18 ban was introduced.
"But supervision, particularly in England, needs to improve to adhere to the legislation which is designed to protect young people from the harms of UV damage.
"Businesses that fail to check the age of their customers are putting young people at risk of developing skin cancer later in life, as well as breaking the law. We must do more and ensure that clear health information is given to those adults who choose to use them.''
Sara Osborne, Cancer Research UK's head of policy, added: "It's encouraging to see fewer children are using them. Rates of skin cancer are disproportionately high in younger people, which is why it's so important to protect the skin during our early years.
"With many children saying they aren't being told about the risks or asked to prove their age, it's vital the Government takes action to bring England in line with the rest of the UK.
"Cancer Research UK urges people not to use sunbeds for cosmetic reasons and we urge all businesses selling sunbed sessions to show responsibility and ask any customer they suspect is under 18 for proof of identity.''
MPs have recently called for unmanned tanning salons to be banned in England.
The All Party Parliamentary Group on Skin said that tanning facilities where people can just turn up and use sunbeds without supervision should be prohibited.
Figures from Cancer Research UK showed more than 13,000 people are diagnosed with malignant melanoma, the most serious form skin cancer, every year compared with 1,800 in 1975.
Malignant melanoma is now the fifth most common cancer in the UK and more than 2,000 people die from the disease each year, a spokesman said. While better detection methods have contributed to the rise, the charity blamed the problem on a "dramatic explosion'' of package holidays and the "boom'' in sunbed use.
A Department of Health Spokesperson said: "We know that using sunbeds significantly raises the risk of skin cancer, particularly in younger people.
"Laws are in place to prevent under 18's from using sunbeds in commercial premises and businesses can be fined up to £20,000 for not complying. Local authorities are responsible for enforcing this.''