Shoppers will be able to compare the environmental impact of imported fruit and veg with that of homegrown ones after Tesco said it was to trial the Carbon Trust's carbon footprint label on 30 own-label lines.

The products in the pilot fall into five categories - including tomatoes, potatoes and orange juice - meaning items that are similar but produced in different ways will be comparable for the first time.

In initial trials the label has appeared on different flavour varieties of Walkers crisps but not on rival brands. "We will be making comparisons between similar products produced under different growing conditionon and with different processing routes, such as air freighting," said Euan Murray, general manager of carbon footprinting at the Carbon Trust. "We will also compare a concentrate orange juice with one that isn't, and a chilled juice with an ambient variety."

Other products to be profiled include light bulbs and washing detergent. The footprint of all 30 products is expected to be calculated by early next year.

Murray said a new carbon measuring methodology, outlined in a draft standard recently published by BSI, had speeded up the footprinting process and reduced costs. Last month we revealed how earlier this year it had taken Boots two months and £40,000 to work out the carbon footprint of just one shampoo.

Tesco had its own scientists based at its new Sustainable Consumption Institute in Manchester to carry out the work on the 30 products in the trial, which would bring the cost down further, said Murray.

The agreement with Tesco is a huge boost for the Carbon Trust. Earlier this year, Tesco chief executive Sir Terry Leahy said the carbon footprint label scheme "did not go far enough". But this week, he said: "While there's still a long way to go, mapping the carbon footprint of these few products will yield invaluable data that will benefit all those retailers and producers who are working towards combating climate change."