Thousands of people are putting their health and life savings at risk to travel to private clinics around the world for unproven and potentially dangerous stem cell treatments, British experts said.

A panel of specialists highlighted individual clinics in Germany and China where so-called "stem cell tourists" go for unlicensed treatment, and said there may be up to 700 similar businesses globally offering unproven cell therapies.

Despite a lack of scientific evidence that such therapies work, patients whose lives are blighted with conditions like Parkinson's disease or childhood blindness are being lured into spending tens of thousands of dollars with little chance of success.

"The patient is in danger of losing their life and health, needlessly traveling long distances away from home, friends and family, not having their condition improved, and potentially losing a large sum of money," said Chris Mason of University College London's (UCL) regenerative medicine bioprocessing unit.

The scientists cited one case of an Israeli boy who received a stem cell treatment in Russia for a spinal injury and subsequently developed multiple tumors.

In another case, they said, a 46-year-old woman was treated in Thailand for the autoimmune disease lupus. She later developed kidney failure and died from sepsis.

Stem cells are the body's master cells that give rise to many different tissues and blood cells. They are standard treatments for leukemias and a few other genetic diseases, but their use in treating other conditions such as Parkinson's, spinal injury or optic nerve damage is as yet unproven.

The International Society for Stem Cell Research has previously warned of rogue stem cell clinics around the world seeking to exploit desperate patients oblivious to the risks.

The British experts said they had been prompted to speak out because of a flood of requests they get from patients who read about apparently dramatic cures on websites and in the media.

They said that while private clinics were not operating illegally, they were offering treatments that are unlicensed and in many cases untested, and advised patients to steer clear.

Treatments can cost around $30,000 a patient, they said, and in the absence of clinical trial data to support their work, clinics post patient testimonies on their websites from people who say they have been helped.

Health authorities in Costa Rica ordered the country's largest stem cell clinic to stop offering treatments in June, saying there was no proof they were effective.

Thailand and Mexico also offer stem cell treatments.

The British experts said they were particularly concerned about a firm in Germany called XCell-Center and a firm in China, Beike Biotechnology, which offers stem cell treatments for a range of conditions including brain injury, cerebral palsy, multiple sclerosis, spinal cord injury and optic nerve damage.

XCell said it would issue a statement on Wednesday about the experts' comments and Beike did not immediately respond to a Reuters request for a response.

"These companies do not offer licensed therapeutics, so it is something the patients themselves have to sign consent forms for," said Peter Coffey of UCL's ophthalmology institute, referring to the two named firms and others like them.

"The main issues around licensing a therapeutic is not just that it's safe... but also to show efficacy. None of these companies has ever been through that type of procedure."

Robert McLaren, a consultant at Moorfields Eye Hospital in London who was also on the panel, said there was "categorically" no evidence to date that stem cell therapies could effectively treat retinal or optic nerve conditions.

"Unfortunately we are dealing with patients who are completely desperate and willing to try anything," he said.