The true scale of the drive to apply carbon footprint labelling to thousands of products in supermarkets was thrown into sharp relief this week when Boots admitted it took two months and £40,000 to work out the carbon footprint of just one shampoo.

Speaking at The Grocer's Green Issues conference on Wednesday, Andrew Jenkins, Boots' sustainable development manager, said the high street chemist's involvement in a Carbon Trust trial to measure the carbon emissions generated in producing and selling the shampoo had highlighted just how difficult and large the task was.

"Measurement is a very complex process," he said. "It took us a long time and a substantial investment."

His concerns were echoed by Nick Monger-Godfrey, head of corporate responsibility at John Lewis, also speaking at the event. He said the methodology behind the Carbon Trust's label was "complicated", and the process "too time- consuming". "John Lewis stocks 350,000 products and has 6,500 suppliers," he said. "Calculating the carbon footprint of all of these is a massive task and it's a thankless task because you are constantly having to re-calculate and re-communicate. We need to be pragmatic about the amount of time we put in, but if we take a quick and dirty approach, so to speak, to calculating, how can we ensure it is accurate?"

Monger-Godfrey said consumers were not even demanding this information and he could not see any financial advantage to being a first mover with the scheme. "I'm not convinced consumers are prepared to pay more for products with low carbon emissions," he added.

However, Euan Murray, who is leading the Carbon Trust's carbon footprinting scheme, insisted that although start-up work could be time-consuming, applying the labelling became simpler as more products were included.

"Once we had gone through the process of working out the carbon footprint of Walkers cheese and onion crisps, it took just three days to do it for all other flavours," he said.

Also speaking at the conference, Tesco business development director Barney Burgess said Tesco was supportive of finding a methodology to measure carbon emissions, but insisted it had no plans to put any pressure on suppliers to become more environmentally friendly."We just want to present the information," he said. "Our customers will let us know if requiring suppliers to reduce emissions is important."

Marks & Spencer told delegates retailers needed to ensure they were able to live up to any green pledges they made.

"We were given the option of signing up to 100%-renewable energy, but we decided to go 50% for two years because we don't want to be held to ransom if prices rise," said Mike Barry, head of CSR at M&S.

"It would be very bad PR to go back on ourselves once we'd announced we were using 100% renewable energy only."

n The presentations given at the Green Issues conference are available to buy on CD-Rom for £175. To order your copy call 01293 610391.