People with Parkinson's disease or childhood blindness are being tempted into spending tens of thousands of dollars with little chance of success. They are also “in danger of losing their life and health, needlessly travelling 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 bio-processing unit.

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

“These companies do not offer licensed therapeutics, so it is something” for which “the patients themselves have to sign consent forms," Peter Coffey of UCL's ophthalmology institute said.

“The main issue 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.”

For Robert McLaren, a consultant at Moorfields Eye Hospital in London, there is "categorically" no evidence to date that stem cell therapies can be used as treatment.

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

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