Gerald Schatten, of the University of Pittsburgh, said he would no longer work with the cloning pioneer Hwang Woo-Suk, following allegations that eggs were taken from a junior scientist in violation of rules meant to prevent coercion.

Professor Hwang gained worldwide attention after announcing last year that his team at Seoul National University had cloned the world's first human embryos and extracted stem cells from them. In May, he announced he had created the world's first embryonic stem cells that genetically matched injured or sick patients - a huge step in the quest to grow patients' own replacement tissue for treating diseases.

But last year rumours began to circulate that the eggs Prof Hwang used in his experiments had been obtained from a junior scientist in his laboratory. Given the health risks - which are relatively minor - involved for women donating eggs, there are strict ethical guidelines as to how eggs should be collected. One of the rules says that senior staff should not collect eggs from junior staff so as to avoid even the perception that the donations were in any way the outcome of coercion.

Dr Schatten told the Washington Post that Prof Hwang had repeatedly denied the rumours and he had believed that position until late last week. "I now have information that leads me to believe he had misled me. My trust has been shaken. I am sick at heart. I am not going to be able to collaborate with Woo-Suk."

In a statement released over the weekend Dr Schatten added: "My decision is grounded solely on concerns regarding oocyte [female gamete] donations in Dr Hwang's research reported in 2004." He said he still believed the team's work constituted "landmark discoveries accelerating biomedical research".

Prof Hwang could not be reached for comment at the weekend.

George Daley, a researcher at the Harvard Stem Cell Institute, and at Children's hospital, Boston, who is due to meet Prof Hwang this month to explore possible scientific collaboration, said: "The National Academy of Sciences guidelines for stem-cell research prohibits payment to egg donors, and scientists in the US have embraced those principles. There is a right way and a wrong way, and we must be sure to perform this vitally important medical research the right way."

Last month Dr Schatten and Prof Hwang announced a not-for-profit collaboration that would have led to at least two human embryo cloning laboratories in the US and in Britain.