By BBC News Online science editor Dr David Whitehouse

Science must play a leading role in restoring public confidence on food safety, says a new report issued by the UK Government Chemist Dr Richard Worswick.

And on the controversial subject of genetically-modified (GM) food, he says that it is impossible to prove that any processed food is free of GM material.

Dr Worswick is chief executive of the Laboratory of the Government Chemist, an independent laboratory that advises the UK Government and others about analytical science.

The 1999 Annual Review of the Government Chemist says that the debate about GM food is "the first science-based debate in the era of spin." It is critical about the standard of scientific knowledge displayed during the GM controversy, and takes to task politicians, pressure groups and the media.

Public debate

The report says that a greater understanding of science and what modern analytical techniques can do would raise the level of the public debate in areas such as on food safety, consumer protection and the environment.

Parts of the report will be worrying reading for those concerned about GM materials in foodstuffs. It says that "no-one should pretend that there can be an assurance in analysis of 'GM free'." It says clearly that no proof of absence of modified DNA is possible.

This is because the processing of foods can destroy the modified DNA to such an extent that no currently-available test is able to detect it. The more processed the food, the more difficult it is to say with any certainty that it was made from GM materials.

False negatives are all too easy to obtain, especially in processed foods. False positives can also occur because of contamination by foreign DNA.

Unenforceable levels

The report emphasises that the best that can be assured is that the DNA in question is not present above a certain level. It adds that because of this, the food industry can only regulate itself at arbitrary and unenforceable levels.

It is also critical of the variations between laboratories when testing for GM materials. "The current inter-laboratory variation of results with GM food does nobody any good," the report says.

Dr Worswick added: "The GM food debate provides a salutary reminder of how the words 'scientists have shown' can give media reports a spurious authority. No-one should pretend that food labelling claiming a product is GM-free can be reliable, since the current variation in results between laboratory tests on GM foods is too wide."

But scientists do face a dilemma. Research into public attitudes to science suggests that telling the public about the detail of new technology does little to increase the society's confidence in it. So what are they to do?