One of the world's leading experts in stem cell research resigned all his government posts today after admitting that he had used eggs from two female employees in his research.

Hwang Woo-Suk, a leader in the quest to use cloned human cells to treat incurable diseases, who is seen as a national hero in South Korea, also gave up his job as the head of the World Stem Cell Hub, a laboratory founded just last month in Seoul to lead research into the use of embryonic stem cells to cure degenerative diseases.

The scientist, a veterinarian, had been besieged by rumours of unethical egg collection after his American collaborators dropped out of the project earlier this month. Dr Hwang's resignation is likely deter further international co-operation and set back his research into Parkinson's and diabetes.

"I am very sorry that I have to tell the public words that are too shameful and horrible," Dr Hwang, 52, told a press conference today. "I should be here reporting the successful results of our research, but I’m sorry instead to have to apologise."

Dr Hwang became one of the world's best-known scientists in February 2004 when his laboratory produced the world's first cloned human embryo. His team also developed the first patient-specific embryonic stem cells, designed to give a precise DNA match for patients.

Earlier this year, Dr Hwang made Snuppy, the world's first cloned dog.

But today he admitted that his rush to produce usable stem cells had clouded his judgement. "Being too focused on scientific development, I may not have seen all the ethical issues related to my research," said Dr Hwang.

Dr Hwang used 242 eggs collected from 16 women to create his human embryo in 2004, and claimed that the ova came from unpaid and voluntary donors. Today, he told reporters that two of his researchers had provided eggs in 2002 and 2003, even though he had turned down their offers to do so.

Under commonly observed international guidelines, scientists are advised against using tissue from humans who are in a dependent relationship with them for research. The guidelines aim to prevent exploitation.

Dr Hwang said he first found about the egg donations in May 2004, when he was asked about the provenance of his material by a reporter from Nature magazine. Dr Hwang said he made inquiries, found out about the donations but then lied to protect the privacy of his researcher.

"The responsibility for all disputes and controversy lies on me," Dr Hwang said today, adding that he will continue research at Seoul National University. "I will not make any excuse."

Mark Henderson, science correspondent for The Times, said Dr Hwang's offences were not serious but that they would cause consternation among scientists trying to work delicately in a highly controversial field.

"People are so sensitive about stem cell research, it's controversial enough already and I think there is quite a strong feeling in the scientific community that researchers should take extra precautions, especially when procuring eggs for research. I don't think Dr Hwang has done anything illegal but the use of eggs from junior members of his staff certainly raises the possibility of coercion."

"It's important to remember that none of this invalidates his science, which is still first class. But what this will do is make international researchers, particularly in the US and the UK, very much more reluctant to work with him and share his expertise. Dr Hwang has developed several lines of stem cells, more than anybody else, but this will make people wary of using them and that will set back the whole field of research."

Earlier today, South Korea’s Health Ministry confirmed that a doctor in Dr Hwang's laboratory had also paid up to 20 women to provide eggs for his research, but that Dr Hwang had only recently found about it. Dr Hwang said today that he had asked the doctor about the source of the eggs but was not told about their provenance.

Dr Hwang's research and the relative lack of bureaucracy surrounding scientific testing in South Korea has made the country a centre for innovative stem cell research. Recently, the government commissioned stamps celebrating Dr Hwang’s work, showing a man rising from a wheelchair to walk.

The Health Ministry said today that Dr Hwang had done nothing illegal. A new law banning the commercial trade of human eggs and tightening South Korea's bio-ethics was passed earlier this year.

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