This suggests that even apparently normal clones may have subtle aberrations of gene expression that are not easily detected in the animal clone

Rudolph Jaenisch, Whitehead Institute for Biomedical Research

The warning comes as a group that wants to clone a baby announced that it was close to achieving its goal.

The first human clone is likely to be produced "very soon", the scientist leading the project told an American newspaper this week.

Brigitte Boisselier, the director of company Clonaid, told USA Today newspaper that the project was on schedule and the first clone would be produced in the near future.

But she refused to be drawn on when that was likely to be and what stage the project was at.

Risky technology

Scientists say cloning techniques are still too underdeveloped to attempt on humans.

Experience with animals has shown that most clone pregnancies fail, or result in offspring being delivered stillborn or deformed.

The new research, published in the journal Science, attempts to explain why.

Two leading animal cloning laboratories in the US used mice to investigate what goes wrong during the cloning process.

Rudolph Jaenisch, of the Whitehead Institute for Biomedical Research, Cambridge, Massachusetts, and Ryuzo Yanagimachi's laboratory at the University of Hawaii, Honolulu, produced mice clones from embryonic stem cells - the "master" cells that go on to form all the different tissues in the body.

Even seemingly healthy clones like Dolly could have subtle genetic abnormalities

They found that the mice clones had subtle differences in their genetic make-up.

Something was going wrong in the regulatory process that controls whether certain genes are switched on or off in the cells used for cloning.

"This suggests that even apparently normal clones may have subtle aberrations of gene expression that are not easily detected in the animal clone," said Professor Jaenisch.

The cloning expert warned recently that attempts to clone human beings at the current time were "dangerous and irresponsible".

He was joined in his call by British colleague Ian Wilmut, one of the scientists who produced Dolly the sheep, the first adult mammalian clone.

Commenting on the new research, Professor Wilmut said it illustrated the dangers of "copying" people.

He told the BBC: "The most likely outcome of any attempt to do that would include late abortions, birth of children who would die, and worst of all the birth of children who would survive but would be abnormal."

Stem cell work

He said the new work provided the first direct evidence that clones were genetically different from normal animals.

Cloning is still a somewhat imprecise and variable procedure and this is the universal experience in terms of cloning mice, cattle, sheep and goats

Ian Wilmut, Roslin Institute

"Cloning is still a somewhat imprecise and variable procedure and this is the universal experience in terms of cloning mice, cattle, sheep and goats," he added. "There is an unusual pattern of loss of foetuses and unfortunately the death of offspring that are produced by cloning.

"This is the reason why lots of labs, including our own, have called for a moratorium on the use of cloning with humans on safety grounds alone."

Professor Wilmut said the work also had implications for research on stem cells. Many scientists believe that stem cells could one day provide replacement tissues for transplant, allowing patients to "grow their own" brain, heart or muscle cells.

The fact that stem cells were genetically unstable in mice suggests that the same might be true in human stem cells. If this is the case, the use of such tissues for medical treatments could have unpredictable effects, or even cause cancer.