The troubled scientist behind the world's first human cloning experiments has now come under pressure to show that his pioneering research is not flawed.

Hwang Woo-Suk, of Seoul National University in South Korea, may withdraw a key scientific paper published in the journal Science, where he claimed to have produced individual colonies of stem cells from cloned embryos derived from donors.

Professor Hwang has already admitted to the unethical practice of using eggs from his own female co-workers as a source of the stem cells, despite repeated denials when he had been challenged about it in the past.

According to a close collaborator of Professor Hwang, the South Korean scientist has now admitted fabricating data that formed an essential part of the stem cell research published earlier this year in Science.

"Professor Hwang admitted to fabrication," said Roh Sung-il of MizMedi Hospital in an interview yesterday with a Korean television station. Dr Roh said he heard from the professor yesterday morning that there were also no embryonic stem cells remaining from the experiments because all colonies have since died in the laboratory. "I heard some things that I wasn't aware of when I visited Professor Hwang at his request, that there are no embryonic stem cells," he said.

Dr Roh, who was one of the co-authors of the study published last June inScience, said that Professor Hwang had agreed to ask the journal to withdraw the paper because of doubts over the authenticity of the data. An official at the journal said it has not so far had any communication from Professor Hwang, who was unavailable for comment yesterday.

Seoul National University has launched an investigation into Professor Hwang's study, which claimed to have produced 11 lines of stem cells derived from human cloned embryos.

According to New Scientist magazine, several cloning scientists have now demanded that independent tests should be carried out to confirm whether or not the 11 stem cell lines genetically match the patients from whom they are supposed to have been derived. "He should send samples to independent scientists outside the country and lay questions to rest," said Keith Campbell of Nottingham University, who was part of the team that cloned the sheep Dolly.

Science has until now stood by the main findings of the South Korean research even though the scientist has admitted errors, such as duplicating some photographs. According to New Scientist, in one case one of the two duplicated photographs is enlarged relative to the other. In a second, one of two duplicated pictures is distorted by being enlarged to different extents along its horizontal and vertical axes.

"This is a level of error beyond sending the wrong file," Robert Lanza of the cloning company Advanced Cell Technology in Worcester, Massachusetts, told the magazine. "At issue is not the cloning technique, which has been demonstrated in other mammals and which I think we all believe will work, but the published research," Dr Lanza said.

Professor Hwang has been treated as a South Korean national hero and the latest doubts about his research's authenticity will not help his fall from grace.