When President Bush addressed the United Nations last month, he made clear his support for an international ban on all forms of human cloning. But ironically, if the U.N. General Assembly adopts a similar position this week, efforts to outlaw human reproductive cloning across the world could be seriously undermined.

Recently a U.N. legal committee reconvened discussions, initiated four years ago, on a convention on human cloning. Two rival proposals are being considered. One, drafted by Costa Rica and backed by the United States, would introduce a blanket ban on all human cloning. The other, put forward by Belgium and supported by Britain, would prohibit human reproductive cloning but would allow individual countries to make their own decisions about therapeutic cloning for research.

If the Costa Rican proposal is endorsed by the General Assembly, countries that already permit therapeutic cloning under regulation, such as Britain, China, South Korea, Japan, Finland and the Netherlands, will not be able to sign on to the convention. With these nations indicating they will not comply, many others might choose to follow suit, and the prospects for an effective worldwide ban on human reproductive cloning will be remote.

There is no scientific reason to link reproductive and therapeutic cloning. Although they would both involve essentially the same initial technology to produce early human embryos, the subsequent aims would be different. In reproductive cloning, the embryo would be implanted in the womb to bring about a full-term pregnancy and the birth of a child. But in therapeutic cloning, the early embryo would never develop beyond a microscopic ball of cells in the laboratory, and it would be used to explore new stem cell treatments for diseases such as diabetes and Parkinson's that avoided the risk of rejection by a patient's immune system.

An overwhelming international consensus exists in the scientific and medical communities that an attempt at human reproductive cloning would be scientifically unsound and medically unsafe. Experiments in animals highlight the dangers that reproductive cloning would pose to both fetuses and mothers. And the majority of the public believes it would be unacceptable. Hence, a worldwide ban on reproductive cloning would enjoy almost global support among scientists, doctors and the public.

There also appears to be no legal reason why the United Nations should seek to create the straitjacket of a convention against both reproductive and therapeutic cloning. Britain has shown that it is possible to treat them separately in law. In 2001 it introduced primary legislation against reproductive cloning. This action was taken after it had it had extended the existing rules governing licensed research on early human embryos to include therapeutic cloning. These measures were taken after wide public debate and were passed by majorities of more than two to one in both houses of Parliament.

The United States does not have primary legislation against either reproductive or therapeutic cloning. But by backing the Belgian proposal at the United Nations, it would retain the freedom to ban both and help to make reproductive cloning illegal worldwide.

Fewer than 40 countries have national laws forbidding human reproductive cloning, and a handful of people claim to be taking advantage of this lack of a united front. Just last month Panayiotis Zavos, a fertility expert based in Kentucky, held the latest of a series of news conferences in London to describe reproductive cloning experiments that he says he is carrying out in the Middle East.

Although his claims are not yet backed by solid evidence, they are still understandably causing concern among scientists, doctors and the public. The United Nations could respond effectively to these worries if the United States and other countries backed the Belgian proposal for a convention that bans reproductive cloning but lets individual nations make up their own minds about therapeutic cloning.

The writer is president of the Royal Society, Britain's national academy of sciences.