For decades, British supermarkets have fought to provide the cheapest baked beans, knickers and fish fingers. But this year's battle on the high street is for the "green pound", and environmentalists have declared one store the winner: Marks and Sparks.

Campaigners for Greenpeace and Friends of the Earth say the once-staid high-street chain is now Britain's most ethical and environmentally conscious retailer. Marks & Spencer has outstripped larger competitors by selling "fair trade" cotton T-shirts, coffee and fruit, putting organic cotton baby clothes on its shelves, phasing out GM animal feed, and selling the most ocean-friendly fresh fish.

The country's three biggest retailers, Tesco, Sainsbury's and Asda, are spending millions to win back control of a market for organic, fair trade and wildlife-friendly produce that is worth £2bn a year.

Sales of organic foods are expected to total £1.6bn for last year, but competition has already led to a shortage of organic milk, forcing several retailers to buy supplies from abroad last winter.

Environment campaigners warn that the sudden conversion of supermarkets to the green cause could mean a shortage in ecologically sound fish supplies, leading to steep price rises.

The top four supermarkets, Tesco, Sainsbury's, Asda and Morrisons, have dropped endangered fish species after a damning report by Greenpeace last year. Marks & Spencer and the second greenest retailer, Waitrose, are also promising to find supplies of ecologically sound fish.

Blake Lee-Harwood, campaigns director at Greenpeace, said the decision by Asda and Morrisons to go "green" would transform the high street. "There's a mixture of real movement and tokenism going on at present, but they can't go back. They've made these promises and they're stuck with them."

Additional research by Louisa Dennison

GREEN RATINGS

1. Marks & Spencer

Green rating: *****

Sells most organic, fair trade and eco-friendly goods.

2. Waitrose

Green rating: **** ^1/2

Large range of organic, fair trade foods.

3. Sainsbury's

Green rating: ****

Most "green" of big four.

4. Tesco

Green rating: ***

£100m on green policies.

5. Asda

Green rating: **

Slow to go green.

6. Morrisons

Green rating: ^1/2

Has fewest organic and fair trade goods.