"We're seeing a lot of knee jerk responses," Leahy told reporters on Wednesday, announcing Tesco's 25 million pounds (US$50.89 million) fundingover five years of a new "Sustainable Consumption Institute" (SCI) at the University of Manchester.

"We can pose these questions to the SCI."

The United Nations Environment Programme (UNEP) has dimissed as too simplistic some western consumer calls not to buy certain air freighted food products.

Blamed for global warming, carbon dioxide is the commonest greenhouse gas and is a by product of burning fossil fuels, and the air freight issue hinges on whether energy-intensive farming in rich nations cancels out the greenhouse gas emissions of flying products from Africa instead.

"It may be that farming from further afield is actually environmentally better, we'll have to wait and see the numbers," said Leahy.

Tesco said in January that it planned to tell its customers about the greenhouse gas emissions of all its products through carbon labelling.

It "will take years" to achieve that across its whole range, Leahy said on Wednesday, while adding that some carbon labels would appear in early 2008.

Other unwanted trade-offs in the response to climate change have emerged from the use of biofuels, derived from plants as a transport fuel to replace gasoline and diesel.

In Britain, Tesco's fleet of lorries runs on fuel which comprises 50 percent biodiesel, while it sells a 5 percent biofuel blend to its customers.

But in a report called "Biofuels: is the cure worse than the disease?", the Organisation for Economic Corporation and Development (OECD) said on Tuesday that biofuels could cause worse problems than the fossil fuels they replace.

Those problems included raising food prices by using land and crops formerly used to produce food, and increased pollution where biofuels were derived from formerly forested land cleared by slash and burn.

Tesco hopes to cash in on rising "green" awarness among consumers, using its economies of scale and marketing might.

"We've got to get a way to use mass marketing techniques in green consumption because people can't pay more," said Leahy.