An inquiry has been launched by health and trading standards officials after suppliers were found selling bogus cancer cures to patients.
Only registered medical practitioners can treat cancer
A BBC Wales investigation found suppliers in south Wales advertising herbal remedies for cancer over the internet.
It is illegal in the UK for anyone other than registered medical practitioners to claim they can cure or treat cancer.
But undercover journalists from BBC Wales documentary programme Week In, Week Out received a consultation from Jim Wright, a supplier of alternative herbal products, before buying his remedies.
Mr Wright, of Port Talbot, south Wales, advertises his "miracle cures" through an internet web site.
The Medicines and Healthcare products Regulatory Agency (MHRA), which regulates the licensing and advertising of medicines, and trading standards officials in Neath Port Talbot have launched an investigation into the activities of Mr Wright.
Week In, Week Out presenter Louise Elliott confronts Jim Wright
Posing as a cancer patient, a Week In, Week Out journalist was told by Mr Wright that he had various herbal products which could cure the disease.
Mr Wright also claimed to have cured himself of skin cancer.
He said of one of his herbal remedies: "It attacks the root, draws it out and that's the end of it."
But Malcolm Mason, professor of oncology at the University of Wales College of Medicine in Cardiff, said of the claims: "That would be such startling news that it ought to be broadcast loudly from every corner of the globe.
"I don't believe it exists."
When questioned over his activities, Mr Wright said he was simply helping people.
Professor of oncology Malcolm Mason has criticised bogus herbal remedies
During the investigation, a self-styled therapist from Swansea also claimed he could help cure cancer and Aids using a detoxification programme.
Trading standards officials in Swansea have now launched an investigation into the man's activities.
Vic Perfitt, of the British Herbal Medicine Association, said he was worried that such practices risked discrediting genuine alternative practitioners.
"He's breaking the law and needs to be stopped," said Mr Perfitt.
"To claim that this would treat cancer is preposterous - it brings herbal medicine into disrepute."
Week In Week Out, BBC 1 Wales, Tuesday, 20 May, 1035 GMT.