Scientists and politicians in the United States have condemned President Bush for blocking embryonic stem cell research that could one day save lives.
By James Westhead
BBC News, Washington
It was President Bush's first veto in six years in office
It is the first time Mr Bush has refused to sign a bill approved by Congress. He said the research which uses frozen embryos was morally wrong.
Democrats described as "shameful" a decision which is out of step with public opinion in America.
Mr Bush announced his veto in a carefully-crafted White House media event surrounded by so-called "snowflake" children, born from frozen embryos which - but for the parents who adopted them - would have been discarded after fertility treatment.
"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 decent society needs to respect," the president said.
"Each of these children was adopted while still an embryo and has been blessed with a chance to grow, to grow up in a loving family. These boys and girls are not spare parts."
Earlier the White House went even further suggesting the president equated such research to murder.
The veto - the first during six years of his administration - has delighted the president's core Christian conservative supporters but risks alienating more moderate republicans.
Scientists and senior Democrats sharply criticised the president's decision.
Iowa Democratic Senator Tom Harkin described it as "a shameful display of cruelty, hypocrisy and ignorance".
Several senior Republicans who were won over to supporting the bill expressed disappointment.
Senate Majority Leader Bill Frist, a heart surgeon, was quick to criticise the president.
"I am pro-life, but I disagree with the president's decision to veto the Stem Cell Research Enhancement Act. Given the potential of this research and the limitations of the existing lines eligible for federally-funded research, I think additional lines should be made available."
Politicians from both sides are now trying to calculate the impact of the decision.
A number of lawmakers suggested Mr Bush's stance could hurt Republicans in congressional elections in November.
Democrats are hoping they can use the image of a president who puts his principles before compassion to take votes.
"Mr President, we will not give up," said Sen Edward Kennedy, a leading Democrat.
"We will continue this battle."
Indeed if Republicans lose control of the House of Representatives, the presidential veto could be overturned and the bill passed into law as soon as next year
The American public is certainly against its president on this highly emotive issue.
Opinion polls suggest almost three out of four are in favour of research on embryos if it could save lives. The issue cuts across traditional political and social lines for voters.
One example is Republican and committed Christian Debi Martin from Cincinnati.
Debate over stem cell research is a mix of science, politics and ethics
"This is a vote-breaker for me," she said.
"I tell people I'm becoming a Republi-crat at this point - because there are just things wrong in the Republican Party where people's voices are not being heard any more."
Ms Martin feels strongly because her nine-year-old daughter, Jessi, has diabetes. She hopes stem cell research could one day find a cure.
However, Mr Bush's advisers, including Karl Rove, will have done their sums.
They know that to have failed to veto could have been even more damaging.
It would have alienated his core supporters and appeared an unprincipled "flip-flop" to the wider public.
Some analysts suggest it will in fact help Republicans in the mid-term elections.
They argue although his decision is unpopular with most Americans, it is only a make-or-break voting issue for a minority.
A number of Republicans voted against the president's position and may have calculated that such a stand will help in their individual electoral races.
However, others are at risk over the issue.
Republican Senator Jim Talent, who won his Missouri seat in 2002 by just one percentage point, opposes expanded research and could lose his seat in November amid heavy campaigning by his Democratic opponent in favour of stem cell studies.
The politics of stem cell research is almost as complex as the science itself. But there can be little doubt that this potent mix of emotion, morality and compassion will be part of the political battleground at the next election.