Supermarket chains are tripping over each other these days to stress their environmental credentials. In the words of Asda's Andy Bond, "green is the new black." Even Morrisons - never seen as the most proactive company - changed its daunting black 'M' to a leafy green on its packs.

But the multiples' sudden enthusiasm for everything ecological has elicited an equal degree of cynicism. Can we really take them seriously?

Last month, Sainsbury's doubtless felt it had pulled off a green coup when fashionistas queued outside its stores in the morning to buy up all its stocks of the Anya Hindmarch 'I'm not a plastic bag' cotton carriers. The media was quick to report the contradiction that the cotton carriers were being wrapped in plastic bags at the point of sale. By afternoon, Sainsbury's was splashed over The Evening Standard, accused of hypocrisy. The chairman of the International Development Select Committee and garment industry campaigners alleged that the cotton had been picked, and the bags manufactured, under low pay conditions in China. 

A day earlier, Asda's announcement that it would be running a trial asking shoppers to return excess packaging to stores, was met with scepticism. The idea of Asda staff rummaging through wheelie bins to photo surplus layers of plastic asked to be lampooned, as did Asda's statement that it would "use the evidence to confront manufacturers".

Days later, the Daily Mail featured a packet of Asda's own-label dried apricots, all of which come individually wrapped. The packaging industry pointed out that this was the pot calling the kettle black. "It's the retailers who specify the packaging," said Dick Searle, chief executive of the Packaging Federation.

These latest own goals echoed Tesco's embarrassment last year when it made all its carrier bags biodegradable. Friends of the Earth pointed out that the bags needed exposure to sunlight to biodegrade, and that the majority go to landfill, where they break down into methane, an ozone-depleting greenhouse gas.

Critics have queried the multiples' sudden keenness to address the plastic bag problem, pointing out that hard discounters like Lidl and Aldi have been charging for them for years to discourage use, while the bigger chains have reverted to

giving them away. Might the current emphasis on plastic bags be anything to do with the fact they are made from oil, and oil prices are spiralling? And what happened to the supermarket system of grabbing a cardboard box and using that to take your goods home? 

The carrier bag debate underlines what a dual-edged sword environmental issues are for supermarkets. Get it right and they can win lots of 'greenie' points. But in the stampede to go one better than rivals, rushing out a series of hastily-conceived, often ill-thought-out initiatives, supermarkets can look, at best, confused; at worst, self-serving, opportunistic and vulnerable to accusations of 'greenwashing'.

To date, the supermarkets' green agenda has been driven not by enlightened shareholders or managers, but by consumer groups and non-governmental organisations. Asda's exhortations to customers to return packaging echoed environment minister Ben Bradshaw's statement to the same effect. He doubtless got the idea from the National Federation of Women's Institutes' long-running campaign to dump excess wrappings at the store of origin.

Morrisons, meanwhile, has come under fire for its seafood sourcing policies. Last year it was denounced by Greenpeace as 'the UK's worst seafood retailer', with protesters scaling the roof of its Bradford store.

So what progress are supermarkets really making? Last year, when the National Consumer Council put the top eight UK supermarkets to the test on key green indicators - from stocking seasonal food through to energy efficiency and cutting waste - no supermarket shone. Waitrose came top with a B rating, but its performance was full of contradictions. It did well for selling Marine Stewardship Council-certified fish, but got slated for stocking the highest number of air-freighted fruits. The biggest chains, Tesco and Asda, were D-rated. Overall, the NCC delivered a 'must do better' report card. "While there are improvements in some areas, not one of the supermarkets is doing well on all fronts," it concluded. 

Since then, both Sainsbury's and Tesco have vaunted the greeness of their new-built stores. Tesco says its flagship environmental store at Wick, which has wind turbines, solar cells, rainwater harvesting, cold-air retrieval systems and more, has a 50% smaller carbon footprint.

Set against that, a Sheffield Hallam University study found that large superstores, of the kind usually built by Tesco, are the most energy-inefficient in the retail sector. It calculated that it would take more than 60 corner shops and greengrocers to match the carbon dioxide emissions from one average-sized superstore.

The proportion of Tesco's floor space taken up by Extra format hypermarkets is now three times what it was in 2000, dwarfing environmental gains from smaller, more ecologically-devised new builds.

The anti-supermarket lobby deems the very concept of a 'green supermarket' a contradiction in terms. However eco-friendly supermarkets might like to be, inflicting environmental damage is built into the fabric of how they do business. 

In green thinking, small is beautiful, while nationwide - sometimes global - retailers are the polar opposite. Environmentalists want to move from the industrialisation and globalisation of food production and distribution, towards smaller networks that serve communities. But the legacy of almost four decades of unbridled supermarket expansion means independent shops and small-scale food producers are dwindling. When there are fewer farmers and shops, goods must be transported further and people travel further to buy them. 

This means more emissions from cars, plus more pollution from aircraft, lorries and ships as food is transported globally. Research for Defra suggests that car use for food shopping costs the UK £3.5bn a year in traffic emissions, noise and congestion. One in 10 of all car journeys in the UK is now made to buy food.

When supermarkets latch on to biofuels to field criticism that they are encouraging unnecessary and polluting food miles, they jump into more hot water. According to the campaigning organisation Biofuelwatch, Tesco is a major customer of a biofuel company whose fuels contain increasing amounts of palm oil, soy, and sugar cane, crops linked to rainforest destruction and vast greenhouse gas emissions. 

Can supermarkets ever win? By ignoring ecologists' demands, they are seen as environmental barbarians. If they respond as best they know how, they are accused of too little, too late, or of reacting cynically to quell opposition. When it comes to championing the environment, supermarkets truly have their work cut out.n