The announcement in August 2006 that an unapproved variety of genetically modified (GM) rice had been found at low levels in US long-grain rice sent shock waves through the food industry.

Bayer Crop Science's GM LL601RICE had last been grown in field trials in 2001 and was not intended for commercialisation.

Although two other varieties of Bayer's GM rice have been given approval for commercial growing and use as food, neither of these are yet being grown.

All of these varieties of GM rice have been modified to be tolerant to Bayer's herbicide, Liberty (glufosinate), so farmers can use the weed-killer without harming the crop. How the contamination arose remains a mystery and awaits the outcome of a US Food and Drug Administration inquiry.

The recent rice episode follows a very similar incident in 2005 when an experimental and unapproved variety of Syngenta's GM maize, Bt10, was found to have been grown mistakenly for four years. Errors in the laboratory and poor quality control had led to the mix up.

In 2000, another GM maize, Starlink, made by Aventis (now owned by Bayer), was found in the human food chain when it had only been given approval for animal feed because of concerns about possible allergenicity. Farmers had not known or had not been able to keep Starlink separate from other varieties of maize.

In all these cases, there have been international shipments rejected, product withdrawals and legal cases costing the industry millions of dollars.

Unapproved GMOs

The reason these GM contamination incidents have such far reaching effects is that they have affected commodity crops which are being traded internationally. A GM crop does not only require approval in the country where it is being grown; most importing countries also require GM crops to undergo a safety assessment before they are allowed in.

In Europe, because a GM organism cannot be released without approval, the presence of an unapproved GM crop - at whatever level - is illegal except in special circumstances.

The exception is when a GM organism has been through a positive safety assessment in Europe but before final approval has been given, and only applies if the contamination is at a level of up to 0.5% and is "adventitious or technically unavoidable". Neither LL601RICE or Bt10 maize fall into this category.

Therefore, the sudden widespread appearance of an unapproved GM rice has had a dramatic effect. Its detection has led to product withdrawals in Switzerland, Germany, France, Sweden, Ireland and the UK. Shipments into Europe require certification that they are GM free and Japan has halted rice imports from the USA.

Bayer is being sued by several groups of rice farmers in the USA because of the effects on their markets and other claims will probably follow.

GeneWatch UK and Greenpeace run an on-line register of GM contamination incidents which gives information about all the cases of GM contamination that are in the public domain.

There are now 132 incidents on the register and they show GM contamination can arise at every stage of development - from the laboratory, to the field, to the plate.

It shows that the controls in place are prone to failure and human error is increasingly being shown to take place - people seem unable or unwilling to take the precautions required by the law or commercial demands.

For many in the biotech industry, the fuss caused by GM contamination episodes, such as those from LL601RICE and Bt10 maize, is excessive because they do not believe there is a risk to human health or the environment.

Because the full details of these GM crops are not in the public domain, an independent assessment of claims of safety is not possible.

Food worries

Whether these particular GMOs are harmful or not, their presence in the food chain demonstrates the inability of the industry to maintain separation between GM and non-GM lines.

Bayer, Syngenta and other companies are developing unquestionably more potentially dangerous GM crops that have altered nutritional characteristics, produce therapeutic drugs or industrial chemicals. Like LL601RICE and Bt10 maize, these experimental lines do not exist officially and there are no tests available for them.

To reduce the risk, governments and companies will have to screen crops from high risk countries that grow and trial GM crops.

However, because companies maintain much information about the nature of their experimental GM crops as "confidential business information", screening will only be possible for the genes that are commonly introduced as markers, so the risk of contamination remains.

Governments also need to take the failure to comply with the law more seriously.

The fine for the Bt10 contamination incident in the USA was $370,000 (GBP196,000) - a trivial amount for a company the size of Syngenta. Europe and Japan took no legal action. That has to change if a more serious incident is to be avoided.

Food companies must despair about the poor practice of the agbiotech industry. They have to face the public and deal with product removals and then try to obtain redress.

Insurers are likely to continue to be sceptical about providing cover for the risks arising from the use of GM crops and foods, and large biotech companies probably have to self-insure - something that will require explicit reporting to investors. GM crops still look more of a liability than an asset.

Dr Sue Mayer is director of GeneWatch UK, a not-for-profit group that monitors developments in genetic technologies from a public interest, environmental protection and animal welfare perspective