The growth of the internet has given cancer sufferers the chance to access information about their complaint easily and in the comfort of their own homes, but it has also given the market in complementary medicines an extra boost.
Many products can be bought on-line without a prescription, but most of these products have not been tested properly or may have unseen effects when combined with conventional cancer treatments.
In a survey of 32 cancer web sites, Professor Ernst and research fellow Katja Schmidt, both of the Peninsula Medical School, Universities of Exeter & Plymouth, found that the quality of web sites differed extensively, to such an extent that the public should be advised.
Speaking to reporters on Monday, Professor Ernst warned, "if it sounds too good to be true, it probably is."
The report, published in the Annals of Oncology, rated three web sites as high risk, overtly encouraging patients to reject conventional treatment in favour of complementary medicine, while 16% included content that would discourage conventional care.
"A significant proportion of the web sites are a risk to cancer patients," Ernst told reporters. "There is no good evidence that any complementary treatment can prevent cancer."
In total, the web sites advertised 118 different complementary or alternative treatments for curing cancer, most of which are not supported by scientific evidence, according to the report.
This included adverts for shark cartilage, laetrile (which contains a toxin found in the kernels of apricots and peaches), the Gerson's diet and mistletoe.
The report recommends a public awareness programme, highlighting the large number of sites that contain misleading information on cancer treatments, and a "seal of approval", which could be given to web sites deemed safe and reliable.