The US group, however, had no such trouble and two of the cattle clones have even had apparently normal offspring.

The study's authors warn against seeing the results as an encouragement for the handful of people who want to clone a human being.

'No abnormalities'

Robert Lanza, of Advanced Cell Technology in Massachusetts, and colleagues looked at a cow cloning experiment which led to 110 pregnancies.

Almost three-quarters of the pregnancies aborted spontaneously, far more than with IVF treatment, where a quarter are expected to end before term.

But of the 30 calves that made it to birth, 24 lived to maturity, a figure which compares well with what happens to cows born after conventional reproduction.

"Results of general health screens, physical examination, and immune function were normal for all of the animals, including laboratory analysis of blood and urine, and we haven't observed any of the genetic defects or abnormalities reported in the popular or scientific press," Dr Lanza told the BBC.

Encouraging results

"All of the data collected reinforce the view that these animals are clinically and phenotypically normal," he said.

"These include the calf that failed to develop an immune system and died when just over seven weeks of age, described by Renard and his colleagues.

"Given that 73% of pregnancies ended in abortion and 20% of the calves died soon after birth, this paper surely confirms that it would be irresponsible to attempt to clone a person," he said.

"It also raises the question of whether there should be widespread use in livestock production until the technique is more efficient," he added.

The cow clone study appears in the journal Science.