President Bush has said the stem-cell legislation crosses a moral line
The US House of Representatives has voted to ease restrictions on federal funds for stem-cell research, defying a veto threat by President George W Bush.
The Democratic-controlled House voted by 247 to 176 to pass the legislation, already approved by the Senate in April, and send it to Mr Bush's desk.
There is little chance Congress will be able to muster the two-thirds majority needed to override a presidential veto.
Mr Bush, who last year vetoed a similar draft bill, has vowed to do so again.
Democrats argue that the restrictions are impeding vital medical research.
Upon taking charge of Congress last year, the party vowed to push for easing curbs on federal funding for stem-cell research.
Critics say the research requires the destruction of human embryos, which they consider a human life, and argue alternative methods are showing promise.
Stem cells are immature cells, created shortly after conception, which have the capacity to turn into any kind of tissue in the body.
Scientists hope to use stem cells taken from frozen human embryos to repair tissue affected by disease or injury. They say the research could provide breakthroughs in the treatment of debilitating diseases like Alzheimer's and Parkinson's.
STEM CELL TECHNOLOGY
Key sources for stem cells are adult organs or embryonic cells
Adult stem cells are identified and separated from other cells
Embryonic stem cells are removed from 5-day-old embryos
Cells are manipulated to stimulate them to take on a specific function
Specialised cells may then be used to treat unhealthy areas
Democratic Representative Diana DeGette, who backed the bill in the House, described stem-cell research as "the most promising source of potential treatments and cures" for sufferers of such conditions.
"Unfortunately, because of the stubbornness of one man - President Bush - these people continue to suffer as they wait," she said.
Republican Representative Chris Smith countered that passing the bill could lead to research developments that would eventually "require the killing of millions of embryos".
In a statement issued from Germany, where he is attending the G8 summit, Mr Bush said: "If this bill were to become law, American taxpayers would for the first time in our history be compelled to support the deliberate destruction of human embryos.
"Crossing that line would be a grave mistake. For that reason, I will veto the bill passed today."
The process of extracting stem cells from an embryo destroys that entity; and conservative Republicans have likened the destruction of frozen human embryos to abortion.
Surveys suggest a majority of Americans support expanded stem-cell research.
If Mr Bush exercises his veto, it will be only the third time he has done so since becoming president - and the second time on this issue. His other veto was of an Iraq war funding bill that would have tied funds to a timetable to withdrawal.
Mr Bush imposed restrictions on spending government money on stem-cell research when he came to power in 2001.
He limited the offer of federal funds to research on stem-cell batches that were already available that August and ruled out funding work on fresh batches.
The president has instead offered to fund an alternative form of research which uses cells taken from amniotic fluid, placentas and from embryos that have died naturally.