When the World Stem Cell Hub was launched last month, it promised to be one of the most exciting international collaborations in science.

Woo-Suk Hwang, the Korean scientist who has made a string of breakthroughs in human cloning, had agreed to share his expertise, and his supply of embryonic stem (ES) cells, with centres in Britain and the United States. The partnership stood a real chance of accelerating the search for new treatments for diseases such as Parkinson’s and diabetes.

It is now in disarray. Two weeks ago, Hwang’s highly respected American associate and friend, Jerry Schatten of the University of Pittsburgh, withdrew from the project citing concerns about its ethical standards. A senior colleague, Sung-Il Rho, then owned up to paying women who donated eggs to the programme. On Thursday, Hwang himself resigned, admitting that he had used eggs donated by junior scientists in the Seoul lab - a charge he had denied when it was first made last year.

The revelations are serious because human eggs are the essential raw material of cloning. Much of the Koreans’ success has been attributed to the large stock of high-quality eggs with which they have been able to work. It is now clear that many of these have been obtained in a dubious fashion.

Egg donation carries a risk of ovarian hyperstimulation, a dangerous and potentially deadly complication; and for it to proceed ethically it is essential that women give their informed consent without coercion. Donors must not be induced to gamble with their health out of financial need, or fear that refusal might jeopardise a fledgeling career. There is no doubting the Koreans’ technical accomplishments. But, keen as scientists in Britain, the US and elsewhere are to learn from them, they will not collaborate while concerns about their donation programme remain. ES cell research and therapeutic cloning are already opposed by religious groups who object to the manipulation and destruction of human embryos. It would be most unwise to invite ethical criticism from more moderate voices by taking a cavalier approach to the sources of raw material.

What the difficulties in Korea underline is that if stem cell research is to proceed in line with the moral standards expected in Western nations, these countries have to take responsibility for it themselves.

This has not been a particular problem in the UK, where the Government has taken the far-sighted decision to permit such work under tight regulation. But the same is not true of America, which has much greater resources to invest.

Roll the clock back five years, to before the Bush Administration banned federal support for most ES cell science, and American researchers were actually leading the world in this field.

Bob Lanza of ACT, one of the companies that has done most, this week blamed official hostility for holding back such work in the US, where it would have been conducted with more ethical safeguards. “The absence of a strong American competitor in this research narrows the range of directions likely to be explored,” he wrote in a letter to Nature. Lanza is right. A lack of American involvement has meant the first significant breakthroughs have been made elsewhere, in ethically tainted circumstances that have cast a pall over the field.

If the medical potential of stem cells and therapeutic cloning is to be realised in a way that is acceptable to Western opinion, it cannot be left to other countries where such considerations have lower priority. It is very hard to influence the direction of work you do not control. The sooner American politicians realise this, the better the prospects for patients of all nationalities will be.

• Mark Henderson is the Times science correspondent. This week he won the Norwich Union/Medical Journalists’ Association award for medical news reporting

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