Patients should beware of so-called stem cell wonder cures as most have not been properly tested, experts say.
Medical charities and the parliamentary group governing stem cell research said stem cell therapy offered promise.
But in a letter to the Times they said some foreign clinics had made claims about multiple sclerosis and cosmetic surgery without scientific foundation.
Reports have emerged of patients going abroad and paying over £10,000 for treatment not available in the UK.
Stem cells are the body's "master cells" and have the ability to produce all manner of tissues, prompting claims they could be used to replace the failed cells responsible for many conditions, such as heart disease and diabetes.
But to date only a handful of treatments have been licensed in the UK, principally for treating leukaemia, and eye and skin disorders.
The letter was signed by the UK Stem Cell Funders Forum, which includes the Medical Research Council and over 100 medical charities, and Lord Patel, chairman of the steering committee for the UK Stem Cell Bank and for the Use of Stem Cell Lines.
The letter said: "Extravagant claims have recently been made of success for 'stem cell' treatments available only abroad.
"We advise those who are desperate for cures or attracted to cosmetic therapy to be wary of claims being made by clinics offering these treatments."
It said the detailed results of unorthodox stem cell treatments, such as MS therapy and cosmetic surgery treatment, had not been published or independently reviewed.
And the letter added: "Indeed, there is concern that these unproven treatments could be dangerous, potentially exposing patients to the risk of uncontrolled and inappropriate tissue generation."
The experts said they were prompted to raise the issue because of reports of patients going abroad, particularly to the Netherlands, for treatment not available in the UK.
Professor Colin Blakemore, chief executive of the Medical Research Council and one of the signatories of the letter, said patients who did seek unorthodox treatment risked hampering stem cell research.
"This is a delicately poised field of research, with a difficult ethical background.
"Just one application of maverick stem cell science that leads to cancer could set back the legitimate field by years, if not decades."