Doctors and scientists expressed dismay that Mr Bush chose the Stem Cell Research Enhancement Act, passed by 63 votes to 37 in the US Senate on Tuesday, as the first Bill that he has vetoed since taking office.

Although two thirds of Americans support government funding of embryonic stem cell research, which scientists say could lead to cures for many life-threatening and disabling ailments, including Parkinson’s and Alzheimer’s diseases, spinal cord injuries and juvenile diabetes, Mr Bush has been adamant that he would block such legislation on moral grounds.

In a White House ceremony at which he was surrounded by children whose parents “adopted” frozen embryos that would have been discarded by fertility clinics, he said: “This Bill would support the taking of innocent human life in the hope of finding medical benefits for others. It crosses a moral boundary that our society needs to respect, so I vetoed it.” Mr Bush banned federally funded research in August 2001. Since then US scientists have had only about twenty embryonic stem cell lines, in existence when the ban was instituted, to use for research. Scientists say that most of those are now contaminated or unusable. The Bill would have provided taxpayers’ money for research using new stem cells from human embryos.

Lord Rees of Ludlow, the president of the Royal Society in London, said that Mr Bush’s veto was “slowing down the global effort to develop therapies for a range of diseases and illnesses . . . that could eventually help millions of patients in the US and the rest of the world”.

Sean Tipton, the president of the Coalition for the Advancement of Medical Research, a US group that campaigns for stem cell science, said that Mr Bush had compromised severely America’s leadership in the fields of science and medicine.

Steven Goldman, of the University of Rochester in New York, said: “Of course I am dismayed. We have so much more information now that we could take advantage of.”

Medical research is increasingly reliant on international collaboration, and tough constraints on embryonic stem cell work in the world’s scientific “superpower” will inevitably hamper progress elsewhere, scientists said.

David Macauley, the chief executive of the UK Stem Cell Foundation, said: “This is about realising the potential of stem cell research and this will only be achieved if we encourage and support research and collaboration worldwide.”

Mr Bush supports adult stem cell research. But even some doctors opposed to embryonic stem cell research on moral grounds expressed anger at recent comments by Karl Rove, the President’s chief adviser, that researchers have found “far more promise from adult stem cells than from embryonic stem cells”. Markus Grompe, director of an Oregon stem cell centre, said there was “no factual basis to compare the promise” of the two stem cell sources.

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