Genetically modified potatoes can damage the immune systems of rats, according to British research released today that calls into question the safety of the new food technology.

Professor Arpad Puztai of Aberdeen's Rowett Institute said he had fed five rats genetically modified potatoes that carried genes from the snowdrop and jackbean for 110 days - equivalent to 10 years in human terms.His research showed that the rats suffered from slightly stunted growth and were more likely to be vulnerable to disease. It was thought to be the first time that trials of GM food had showed harmful effects.

Puztai said his results meant that genetically modified crops should be tested much more rigorously before being cleared for human consumption.

"We are assured that this is absolutely safe and that no harm can come to us from eating it. But if you gave me the choice now, I wouldn't eat it," he said.

"We need to be far more careful in devising testing programs. It is expensive, it is long, but nevertheless it is the only way that you will be able to pick up differences.

"We are asking for less haste and more testing," Puztai told BBC radio.

Britain's agriculture ministry said genetically modified potatoes had not been approved for human consumption in Britain, although soya and other some other GM products have been on sale for about two years.

The particular strain of GM potato used in Puztai's experiments are not thought to be sold commercially anywhere in the world.

Monsanto Co., the multinational agro-chemical group that is among the pioneers of the new food technology, said today that all genetically modified food currently on sale around the world was safe and had undergone rigorous trials.

"The safety, both environmentally and the human health safety of these crops, has been well-documented. There have been more than 25,000 field trials conducted on 60 different crops in 45 different countries around the world. . . and not any one of the regulatory agencies in those countries has said that there is a safety issue," a Monsanto spokesman told BBC radio.

Monsanto is currently spending $1.6 million on an advertising campaign in Britain to promote the benefits of the new technology, which it says will make food more plentiful and reduce the need for chemicals in farming.

But many Britons are either skeptical or anxious about meddling with nature to produce food.

Britain's Prince Charles, himself an organic farmer, fueled the debate in June with a warning that genetic engineering of food "takes mankind into realms that belong to God and God alone."