Prof. Puztai tells BBC Radio 5 Live's Breakfast Programme: 'We need more testing'

In an experiment, rats were fed potatoes for 100 days which had been modified to make them resistant to pesticides.

After seeing the results of his work at the Rowett Research Institute in Aberdeen, Professor Arpad Puztai said he would not eat genetically modified food.

The findings, he said, raised grave questions about the safety of the food for humans.

The Rowett in Aberdeen is world-renowned for its food research

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

The most worrying feature of the experiment was the effect of the potatoes on the rats' immune systems, he said.

Conservative health spokesman Alan Duncan: 'Massive forces are being unleashed'

"The system is there to fight off all disease-carrying bacteria that get into our bodies.

"If it is damaged and cannot mount a proper response, then we are in trouble."

A growing proportion of processed food available in Britain contains GM ingredients but there are no legal requirements for warning labels on packaging.

MP wants ban

Liberal Democrat environment spokesman Norman Baker, who disclosed last week that GM food had been removed from the menu at the House of Commons, called for a ban.

"The new health findings are very worrying and show that we have become the guinea pigs in a gigantic experiment.

"The government has been irresponsible and spineless in allowing GM foods into our diet without demanding to see definitive proof that it is safe.

"The only proper thing to do now is to ban GM ingredients from all foodstuffs."

The Ministry of Agriculture, Fisheries and Food said that genetically modified potatoes had not yet been approved for human consumption in the UK and that the department would be interested in seeing the test results.

"The government is confident that each food that has been approved so far is safe," said MAFF.

Top biotech company Monsanto questioned the fairness of the tests as the rats were fed on an unlicensed crop.

Monsanto's Dan Verakis: 'There are very real benefits to this technology'

Spokesman Dan Verakis said that they had carried out "intensive safety assessments of new biotech crops" including tests using rats that have had results published in journals.

Organic farmer Guy Watson lost a legal case recently seeking to prevent GM maize being grown next to his farm in Devon. The crops were destroyed last week by protesters.