Tanning clubs and shops are failing to give proper advice to people who use sunbeds, a study has found.
Sun beds are popular with the young
A Consumer Association investigation also found safety checks are not being carried out - increasing the risk of users damaging their skin.
The report's authors say their findings underline the need for tighter regulation of the industry.
Sunbed use has been linked by various studies over the years to an increased risk of skin cancer.
Researchers from the watchdog's consumer magazine Health Which? went to 42
tanning outlets across the UK in February this year.
Staff were told a friend wanted to use the sunbed who had red hair and very fair, freckly skin that always burned in the sun.
People with this type one skin are among those at highest risk of developing malignant melanoma due to exposure to ultraviolet radiation from the sun and sunbeds.
Nevertheless, 40 out of the 42 outlets visited said they would allow a person
with type 1 skin on their sunbeds.
Just under half the 40 were members of the Sunbed Association whose code
of practice states members should turn away people with type 1 skin.
The outlets which gave bad advice were six run by local authorities, 20 tanning shops, 13 health and fitness clubs and one beauty salon.
Just two outlets - one local authority operated and a beauty salon - were deemed to offer an "adequate" service.
However, neither of these correctly advised customers not to have more than 60 sessions on a sunbed in a year.
Kaye McIntosh, from Health Which?, said little had changed since a similar investigation uncovered the same problem five years ago.
She said: "As the industry has clearly shown that it is incapable of regulating itself,
the government must act immediately on the overwhelming advice from national and
international medical experts and honour its duty to protect the public from serious health risks associated with the use of sunbeds in the UK."
Sara Hiom, science information manager at Cancer Research UK, said: "Laboratory research has proved that the type of ultraviolet radiation used in sunbeds can cause all types of skin cancer.
"And studies are starting to show that sunbed use is a definite risk factor for malignant melanoma."
The Sunbed Association argued the investigation was "flawed".
In a statement it said: "The style of the mystery shopper research, and the criteria for evaluating and grading the service provided, meant that all of the facilities visited - whether in TSA membership or not - were highly likely if not definite to be given an 'inadequate' grading.
"An automatic 'inadequate' grading was awarded to any facility that did not screen out a skin type 1 via a questionnaire."
Cancer Research UK and the Sunbed Association recently called for a ban on sunbed use by children under 16 due to the high risk of cancer in later years.
Cases of malignant melanoma - the potentially fatal form of skin cancer - increased by 24% between 1995 and 2000.
About 7,000 cases of melanoma are diagnosed annually with about 1,700 deaths resulting from the disease.
It is the third most common cancer among people aged 15-39.
More than 62,500 cases of non-melanoma skin cancer are recorded in Britain each year.