Leading scientists have called on the government to support research to create hybrid human-animal embryos.
Early embryos yield stem cells
In a letter to The Times newspaper, the 45 scientists, academics and politicians accuse the government of wanting to ban such research.
The group claims the decision could go ahead without "giving any proper reason or citing any evidence".
Some scientists want to create hybrid embryos as a source of stem cells for finding cures for human diseases.
The research could help in the fight against conditions such as Alzheimer's or motor neurone disease.
Nobel winners' support
The Human Fertilisation and Embryology Authority (HFEA) is meeting on Wednesday to consider its policy in this area.
The letter's signatories include Nobel prize-winners Sir Paul Nurse, Sir John Sulston and Sir Tim Hunt.
Scientists last week accused the HFEA of bowing to government pressure if it fails to consider their applications to carry out the research.
Ministers proposed outlawing such work after unfavourable public opinion.
Supporters say research on hybrids makes sense
PM Tony Blair said any new law would have "flexibility" to support scientific research that helped people.
The government has opposed the creation of human-animal hybrids or so-called "chimeras" - where genetic material is taken from humans and put into a host animal egg.
Opponents have said the research tampers with nature and is unethical, while scientists have called for greater understanding of what they are trying to achieve.
Scientists are hopeful that studies on stem cells - immature cells that can become many types of tissue - could lead to greater understanding and even a cure for many diseases, including Alzheimer's.
They say using human-animal mixes rather than human eggs to get the stem cells makes sense because human eggs are in short supply, plus the process is less cumbersome and yields better results.
'Ill thought out'
Dr Evan Harris, a Liberal Democrat MP on the Science and Technology Select Committee and a member of the BMA Medical Ethics Committee, said the letter demonstrated how "ill thought-through" the government's White Paper had been.
"This is an unprecedented show of unity and support from scientists,
clinicians, ethicists and patient groups, and shows how concerned people are about this vague, ill thought-through proposal from the Government," he said.
"It would be bizarre for the HFEA to decide to ally itself with those who
oppose all embryo research."