"Consumers say they want us to help them behave in a more sustainable way. It seems to be quite a substantial change in customer attitudes," Neville-Rolfe, Tesco's Executive Director of Corporate and Legal Affairs, said.

Tesco labels food for nutritional benefits with a scheme that uses guideline daily amounts (GDAs) rather than the UK government-backed traffic light programme. It does not currently label food for its impact on the environment.

British supermarkets have been divided on labelling schemes with some backing traffic lights which show red lights for high levels of saturated fat, salt or other potentially harmful ingredients.

Others, including Tesco, back GDAs, citing fears that a red light for fat on a pack of cheese, for example, could deter consumption of a product that is an important source of calcium.

"The impact of the scheme that Tesco has adopted has been remarkable," she said, noting the sales of Tesco standard sandwiches, for example, had fallen by one-third while its healthy living range of sandwiches had seen an 85 percent rise.

"These are not marginal changes," she added.

Neville-Rolfe said there would be winners and losers if green labelling took off.

"I think green labels could be a big driver for change," she said.

She noted that such a scheme would have challenges, including seeking to provide a market for goods from the developing world which might be rated unfavourably if they were transported to Europe by air.

Neville-Rolfe also said Tesco had sought to meet demand for local food by encouraging producers to extend the growing season by growing fruit and vegetable under glass in Britain, adding that this method however used more energy.

"There are difficult issues," she said.