Proposals to clone large numbers of farm animals to produce supermarket milk and meat are not inherently wrong, says one of the world's best known scientists - but the practice should only be sanctioned when cloners have improved their record on animal welfare.

US cloning companies are busy making multiple copies of prize breeding cows and claim cloning could soon become an economic way to routinely produce dairy and meat animals.

Cloning currently costs between $15,000 and $25,000 per cow. But prize animals can fetch $40,000 or more, making cloning economic, according to Ron Gillespie of the Massachusetts company Cyagra. "Push the price down to $10,000 and there would be 100,000 animals that it would be economic to clone, and in the $5000 range, millions."

But Ian Wilmut, who led the team that cloned Dolly the sheep, says it is vital that large-scale, controlled farm trials of cattle cloning are carried out before any commercial production of cloned meat and dairy food is allowed. "If companies start marketing this food and there are problems it will bring the whole technology into disrepute," he told New Scientist.

Sceptical public

Wilmut says the cattle cloners "ought to be making systematic comparisons between clones and animals produced by embryo transfer, looking not just at their milk yield but also their health and lifespan."

He also says that in the wake of the genetically modified food furore, cloners will have to prove to a sceptical public that food from clones and their offspring is just as safe and nutritious as conventionally produced food. Even small imbalances in hormones, proteins and fat levels could alter the quality and even safety of meat and milk.

The warning comes as cattle cloners continue to uncover evidence of severe pregnancy complications and defects caused by cloning. The current list includes dramatically oversized calves, enlarged tongues, squashed faces, intestinal blockages, immune deficiences and diabetes, says Jim Robl of the Massachusetts company Hematech.

Even cloned animals that look healthy often have subtle defects or weird physiologies that defy the textbooks. Herds of identical cloned animals would be a "welfare disaster", says Joyce D'Silva of Compassion in World Farming. "There would be a huge loss of genetic diversity with unforeseeable results in terms of animal illness."

Error prone

The Wisconsin cloning company Infigen says it has amassed a huge database of blood tests from its surviving cloned cows. "The data suggested to the vets that some of them should be dead," says Infigen's Michael Bishop.

Meanwhile, at the University of Hawaii at Manoa, Ryuzo Yanagimachi is looking in detail at the activities of genes inside cloned mice. In light of the unpublished work, he says "all cloned babies have some sort of errors".

Britain's Food Standards Agency says that in Europe cloned meat and milk would, like GM produce, be classed as novel foods so sellers would need a special licence to market them. Unlike GM foods, though, there is as yet no legal requirement for cloned produce to be labelled.

A full length version of this story is featured in the 19 May issue of New Scientist magazine

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