Arguing that private research has produced more than 60 genetically diverse stem cell colonies, or lines, that could reproduce themselves indefinitely, Bush said in a nationalized TV address, "I have concluded that we should allow federal funds to be used for research on these existing stem cell lines, where the life and death decision has already been made."

He added that the research would be limited to cells that had already been extracted, and that the government would not support the destruction of new embryos. As a result, research on stem cells extracted from spare embryos left over after in vitro fertilization cannot go ahead if it uses federal funds.

Proponents of embryonic stem cells research criticized Bush for closing the door on obtaining stem cells from embryos that have been created for in vitro fertilization but are not being used by couples. "There will be concern about the limits the president has proposed on research," said Senate Majority Leader Tom Daschle, a Democrat from South Dakota. "The Senate will want to take action."

Stem cells are the undifferentiated precursors to the body's various organs and tissues. Adult stem cells have already begun a partial process of differentiation; those that exist in the walls of the intestines, for example, can turn into the various tissues of the gut, but not into nerve cells or a new heart (though there is emerging research suggesting that adult stem cells can be reprogrammed to develop into different tissues.

Stem cells derived from embryos, by contrast, are said to be pluripotent because they have the potential to develop into every other kind of cell in the body. Research on stem cells is expected to yield an ability to regenerate tissues and even entire organs. Since such regenerated cells can come from the patient's own body, the latter is much less likely to reject these than it is organs donated by another individual.

Around the world, there is fierce debate about the ethics of embryonic stem cell research. The issues are complex, but in general terms proponents point to the huge potential benefit for health, whilst opponents are concerned that creating embryos for destruction reflects a basic disrespect for the sanctity of life.

President's Bush's attempt to split the difference between these fundamentally opposing views seems unlikely to satisfy either side. Lawmakers running the political gamut from Senator Edward Kennedy, a liberal Massachusetts Democrat, to House Majority Whip Tom DeLay, a prominent Texas Republican, pronounced themselves dissatisfied. Kennedy supports the research because he believes that it offers hope for ending human pain and suffering. DeLay opposes it because it destroys human embryos.

These contrasting opinions — which obviously existed before the President's announcement — were always going to provide an emotional backdrop to debates this autumn on a number of bills that are already in the pipeline and aim to establish an embryonic-stem-cell research program. Now the President has raised the stakes, and has effectively told lawmakers what legislation he is prepared to veto if Congress cannot put together the two-thirds majority needed to override a presidential veto.

Legislative maneuvering will come on two main fronts. First, legislation has been introduced in both the House and the Senate to permit research on embryonic stem cells, and more is expected. Senator Arlen Specter, Republican of Pennsylvania, has written a bill that would permit stem-cell research only on embryos that were created for in vitro fertilization but are destined to be discarded.

Second, every year since 1996, Congress has included a prohibition on federal funding of any research that involves the destruction of human embryos. The prohibition expires 30 September 2001, when the government's fiscal year ends. Supporters of research on embryonic stem cells are likely to try to remove the ban this fall when Congress considers the budget for the Department of Health and Human Services. Opponents of the research are expected to fight to retain it.

Additional pressure is coming from fears that American stem cell researchers will be tempted oversees. The UK, for example, will, under very strict conditions, permit research on embryos up to 14 days old for medical research. Until this year such research had to have the aim of improving fertility, but earlier this year the House of Commons voted so overwhelmingly to permit embryonic stem cell research under existing statutes that legislation could have been forced through even if the House of Lords had opposed the idea.

"The 14-day boundary for research on human embryos was established because there is, at that stage, no suggestion of a nervous system, and the embryo is not aware or conscious," said Ian Wilmut, leader of the team in Scotland that pulled off the first-ever cloning of a sheep named Dolly.

In Canada, it is illegal to clone a human being, but it is not illegal to use fetal tissue or human embryos in research. Federal funding agencies allow experiments on embryos up to 17 days old, but each project must be approved by ethics committees.

Many in Congress are now saying that the US must address the need for a comprehensive regulatory framework for human biotechnology. So far, such rules as exist regarding issues like stem cells, germ-line engineering, human cloning and human experimentation more broadly have only focused on the narrow issue of federal funding.

"This was fine in an age when the NIH (National Institutes of Health) funded the vast majority of biotech research. But today, there is a huge private biotech industry, while hundreds of millions of loose research dollars seek all sorts of morally questionable objectives," said Francis Fukuyama a professor of public policy at Virginia's George Mason University.

"As in the case of stem cells, most future developments in biotechnology will be ones in which positive developments will be intertwined with questionable ones," Fukuyama said. "This implies the need for a standing regulatory structure that will be able to make and enforce distinctions between good and bad uses of biotechnology, on broader grounds than the safety criteria currently used by the Food and Drug Administration."