Food retailers will not be able to rely on tests alone to ensure that their food contains no genetically-modified (GM) ingredients. They will have to know exactly where the foods ingredients came from.

This is because in some cases there is no certain way of telling whether processed food once included ingredients that had been genetically-modified.

If the raw, unprocessed crop is available, or if the food has been only lightly processed, then it is relatively easy to tell a GM plant or ingredient from a normal one.

This is done with a standard DNA test called the polymerase chain reaction (PCR) that looks for and copies specific pieces of DNA.

GM tomatoes: Difficult to test

But when foodstuffs are processed the DNA they contain becomes degraded. Also substances that interfere with the PCR reaction can be introduced during processing and cooking.

The Laboratory of the Government Chemist, an independent UK body says that extracting DNA from highly processed food in a form suitable for reliable PCR testing can be "challenging and unpredictable".

Last year LGC scientists warned food manufacturers and retailers that they would be misleading customers if they claimed that their highly processed products were 100% free of GM ingredients based on tests alone.

The lack of a reliable test means that the only foolproof way to ensure that a food is GM free is to find out exactly what it is made from. This will require finding the source of its basic ingredients and ensuring that they are GM free.

If the food is made from soya or maize imported from the United States this may be impossible as GM altered soya and maize are mixed with unaltered crops.

As part of the requirement that GM foods are labelled the UK government has issued a list of suppliers of certified GM free foodstuffs.