President George W. Bush said this evening he used his first veto to block legislation expanding embryonic stem cell research, putting him at odds with top scientists, most Americans and some in his own Republican Party.
"It crosses a moral boundary," Mr Bush said.
Mr Bush acted after the US Senate yesterday approved the legislation, which has also been passed by the House of Representatives. But neither chamber expects to have the two-thirds majority needed to override the veto.
In 2001, Mr Bush allowed scientists to use 78 stem-cell lines then in existence, most of which proved unsuitable for research. But he opposes expanded federally-funded research because it entails destruction of human embryos.