Dr Hwang has been hailed as a hero in South Korea

At least nine of 11 stem cell colonies used in a landmark research paper by Dr Hwang Woo-suk were faked, said Roh Sung-il, who collaborated on the paper.

Dr Hwang wants the US journal Science to withdraw his paper on stem cell cloning, Mr Roh said.

Dr Hwang, who is reported to be receiving hospital treatment for stress, was not available for comment.

Science could not confirm whether it had received a request to retract the paper.

Dr Hwang's paper had been hailed as a breakthrough, opening the possibility of cures for degenerative diseases.

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Cloning



Resignation

Last month, Dr Hwang resigned from his main post as head of the World Stem Cell Hub, after it emerged that some of the eggs used in his research were donated by his staff - in contravention of international guidelines.

Now it is some of the research itself which is being called into question.

Hwang Woo-suk successfully cloned a puppy, called Snuppy

"Professor Hwang admitted to fabrication," Mr Roh told South Korea's MBC television network after visiting him in hospital.

Dr Hwang's paper claimed to have produced 11 stem cell lines from cloned human embryos.

But Mr Roh has said that a scientist working in Dr Hwang's laboratory was pressured into doctoring results to make the embryos look like clones.

Transatlantic doubts

Mr Roh's comments came shortly after the lead US author of the published paper appealed for his name to be removed from the paper, citing doubts over the accuracy of the work.

University of Pittsburgh biologist Gerald Schatten said he recommended that Dr Hwang and all other co-authors ask for a formal retraction.

In a statement, Science said that all authors of a paper would need to agree to make a retraction possible.

The BBC's Charles Scanlon in Seoul says the revelations have sparked a furious debate in the South Korean media.

Leading companies have pulled their advertisements from the television station that first revealed the reported problems with Dr Hwang's work.

Many commentators said it was unpatriotic to challenge someone who had given the country a lead in such a promising new area.

The stem cell technique being pioneered by Dr Hwang could lead to possible cures for diseases including diabetes and Parkinson's.