Sunbeds pose such a risk of causing cancer they have now been moved to the highest level of danger classification, putting them on a par with asbestos, mustard gas and particles from atomic bombs.

The warning from the World Health Organisation will have particular significance in Scotland, where sunbeds are used more than almost any other country and which suffers one of the world's worst levels of skin cancer.

After a meeting of experts in France last month, the International Agency for Research on Cancer (IARC) has decreed that sunbeds should be treated as "carcinogenic to humans", a significant step up from their previous status as "probably carcinogenic".

The findings, released today in The Lancet Oncology journal, provide the most conclusive evidence to date that tanning salons are a direct cause of cancer.

Dr Fatiha El Ghissassi and her colleagues, on behalf of the WHO International Agency for Research on Cancer Monograph Working Group, warned that use of sunbeds could put people at significant risk.

"The use of UV-emitting tanning devices is widespread in many developed countries, especially among young women.

"A comprehensive meta-analysis concluded that the risk of skin melanoma is increased by 75% when use of tanning devices starts before 30 years of age," they wrote.

"Additionally, several case-control studies provide consistent evidence of a positive association between the use of UV-emitting tanning devices and ocular melanoma eye cancer."

"Therefore," the report continues, "the Working Group raised the classification of the use of UV-emitting tanning devices to Group 1, carcinogenic to humans'."

The characteristic gene mutation caused by solar radiation, or ultraviolet rays, has long been put down to UVB - a type of radiation that penetrates only the very outer layer of skin and is strongest at around midday.

UVA, which works itself deeper into the skin and is present in sunlight all day long, had previously been thought of as relatively harmless.

Unlike UVB it is not filtered by glass, and so is the type of radiation most associated with sunbeds.

Now, however, experiments on shaved mice have detected the same mutations and tumours whether UVA or UVB rays are administered, showing that both types of ray can have a harmful effect.

Because of this, the IARC has reclassified the whole spectrum of UV radiation, also including UVC, which is normally filtered out by the stratosphere, in its top category for danger posed to humans.

The new heightened classification means that sunbeds are now in the same league as cigarettes as a cause of cancer, and it may prompt greater intervention by governments and international health authorities to regulate them.

The findings are especially relevant to Scotland's young people, who are among the world's most prolific users of tanning equipment and salons.

Holyrood announced plans last month to raise awareness of the dangers posed by sunbeds, and ministers hope to build on existing legislation.

Under-18-year-olds are already banned from using ultraviolet sunbeds in Scotland, and unstaffed salons - once a serious concern in Scottish cities - are no longer allowed.

Authorities have suggested compulsory warning signs along the lines of those on cigarette packets, and a public consultation is currently under way.

Aside from the greatly increased risk of cancer - a particular problem for younger sunbed users whose skin is not as weathered as the older population - sunbeds are associated with many other maladies.

Side effects can include premature ageing of the skin, cataracts in the eye and short-term pain through blistered skin and heat rash.

Approximately 100 skin cancer deaths are attributable to sunbed use each year, according to Cancer Research UK, and rays from UV sunbeds can be up to 15 times as strong as those from the midday sun.

The charity warned against people using tanning salons to prepare for a holiday.

It said: "A tan offers very limited protection from sunlight or burning. At most, a sunbed tan is the equivalent to a sunscreen with SPF of just two to four - not enough to keep you safe in the sun."

It added: "Unfortunately, using sunscreen or limiting your time on a sunbed will not completely protect your skin from damage and ageing.

"In fact, short periods of intense, irregular UV exposure, like you get on a sunbed, are the fastest way to damage your skin."