The code, drawn up by the IG DHS, the Swiss retailers association, is in part a response to the fierce criticism drawn by some store operators after they stocked genetically modified (GM) food.

"Companies such as Migros and Coop had a very negative experience over GM products," said Christopher Meili, CEO of The Innovation Society, a Swiss company specialising in nanotechnology risk management.

"They wanted to avoid that bad consumer publicity when it came to nanotechnology," Meili told FoodProductionDaily.com.

His company worked with the retailers to assess the potential risks from nanotech products in food and packaging and to draw up the code of conduct.

According to Meili, Swiss consumers do not necessarily have a negative attitude towards nanotech.

"Consumers are not sceptical about nanotech as such. But with food, consumers want to know what it is that they are eating, and to be able to make informed decisions."

That is why, he said, the code of conduct requires food producers to provide information to retailers about any products that contain nanotech particles.

The Swiss retailers have used the definition of 'nano' used by the national government - particles that are 100 nanometers in diameter or less - but Meili stressed that this was just a "working definition" and that it would have to be refined over time.

But he said that setting it this small would probably avoid ingredients firms having to declare their products as nanotech, as products such as vitamins were broadly speaking around 300 nanometers in diameter.

He said that there had been no feedback from - or consultation with - food producers when the code was drawn up, but stressed that the retailers expected to hear from their suppliers in the near future once the code became more widely known.

And Meili suggested that it might be packaging suppliers who would be most badly affected by the code of conduct.

"There are several nano products already widely used in the food packaging sector, and producers will have to provide information on these now."

He suggested that products such as nanosilver - a biocide used in many packaging products for food as a protection against bacteria - could be caught by the new code.

"Nanosilver is used in many products, but nobody really knows what happens if it migrates from the packaging to the food," he said.

The US environmental protection agency in 2006 decided to regulate nanosilver and assess its impact on the environment, but it is not yet regulated in Europe.

Other nano particles used in food packaging include titanium dioxide, which is a UV blocker, while silicon oxide is used a barrier layer.