Sir, We are convinced that stem-cell research is a highly promising area of science offering potential for new methods of treating many serious diseases. We welcome legal acceptance, public and private funding, and international co-operation for a range of stem-cell research.

But we also wish to caution against false optimism and unrealistic claims for as yet unproven avenues of research. It is irresponsible, unjustified and, especially, unfair to patients for researchers to claim without evidence that a refusal to fund, to license or to approve a particular research approach will “delay treatments for incurable illnesses”.

In particular, given the current state of more conventional embryonic stem-cell research, of adult stem-cell research, and of induced pluripotent stem-cell research, there is no demonstrable scientific or medical case for insisting on creating, without any clear scientific precedent, a wide spectrum of human-non-human hybrid entities or “human admixed embryos”.

We therefore question the scientific validity of proposals to create such embryonic combinations currently before the UK Parliament. We note with concern that, though not widely reported, the Bill does not just propose licensing so-called cybrids (99.9 per cent human, 0.1 per cent other species). It also proposes that embryos “created by using human gametes and animal gametes” (50 per cent human, 50 per cent other species) or human embryos “altered by the introduction of one or more animal cells” (ie, any percentage of human material) could be created under licence (UK Human Fertilisation and Embryology Bill 2007-08, Section 4A(5) and Schedule 2. 3(3)).

All such proposals are highly speculative in comparison to established sources of human stem cells, and we remain unaware of any cogent evidence suggesting any might yield significant therapeutic dividend.

As scientists and clinicians actively involved in stem-cell research and regenerative medicine, we do not hold a single common view about the relative merits, ethics and potential of adult v (conventional) embryonic stem cells. But we all believe that extravagant claims regarding the purported merits of human-non-human interspecies embryos are mistaken and misleading, and that such research would damage public confidence and support, to the detriment both of the cause of stem-cell science and, ultimately, of patients.

Prof Neil Scolding, Bristol

Prof Michael Chopp, Detroit

Prof Dr Wolfgang M. Franz, Munich

Professor Alan Mackay-Sim, Queensland

Professor T. John Martin, Melbourne

Dr Rodney L. Rietze, Queensland

Prof Dennis McGonagle, Leeds

Prof Dr Bodo-Eckehard Strauer, Düsseldorf

Professor Gianni Angelini, Bristol

Dr Roger Barker, Cambridge

Dr Maureen L. Condic, Utah

Prof Dr Ursula Just, Kiel

Prof Dimitris Karussis, Jerusalem

Dr Letizia Mazzini, Piedmont

Dr Jean Peduzzi-Nelson, Detroit

Dr Carlos Lima, Lisbon

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