The premature use of stem cell therapy could put many patients at risk of contracting disease, say experts.
Stem cells hold great potential
They warn that without adequate safety measures use of the cells runs the risk of infecting patients with viruses and prion diseases, such as vCJD.
Stem cell therapy is believed to hold huge promise for treating degenerative disorders like Parkinson's.
The researchers, from King's College London, published their concerns in the British Medical Journal.
They include Dr Stephen Minger, director of the King's stem cell biology laboratory, and Professor Peter Braude, an expert in obstetrics and gynaecology.
They warn that science should learn the lessons from disasters, such as the HIV infection of haemophilia patients who received tainted blood transfusions.
The number of human embryonic stem cell lines being grown by scientists has increased sharply in the last two years, the researchers say.
However, they warn that stem cells have not yet been grown in the conditions that would be expected, for instance, for any pharmaceutical product destined for human consumption.
Many potential patients
Despite this, the expansion of stem cell cultures could allow a single stem cell line to be used for many hundreds, if not thousands of patients, they say.
This would exponentially amplify the potential risk of disease transmission from a single infected donor.
By April 2006, all stem cell and IVF laboratories will have to comply with new EU rules governing the use and testing of cells.
But the researchers suggest that expanded stem cells lines should be tested for a variety of pathogens before they are released for use.
Writing in the journal, they say: "The drive to be the first to provide cell lines for therapy could compromise safety for recipients and could lead this technology into the realms of quackery.
"Stem cell therapy needs to be nurtured safely and methodically to provide real benefit to patients in the future."
Safeguards in place
Professor Roger Pederson, an expert in regenerative medicine at Cambridge University, told the BBC News website he was confident that stem cell research in the UK was safe.
"If people go off half-cocked and do research without proper precautions it could endanger patients, but people are not doing that in the UK," he said.
However, he said research of a more dubious nature was taking place elsewhere.
"The rule of thumb here is that if anyone is asked to pay to take part in a trial, then they probably should not do it," he said.
"Any legitimate trials will be paid for by governmental sources."
The issue will be discussed at a public debate in London next week.