We've had a smoking ban, a ban on junk food ads for kids and now retailers are facing the prospect of a ban on the humble plastic carrier bag.

Despite the raft of green initiatives launched by the supermarkets, it is the throwaway bag that has become the most visible measure of their sustainability credentials. So why are supermarkets, which have made so much noise about reducing carrier bag usage, balking at the prospect of an outright ban?

Put simply, they believe they are doing enough already.

The British Retail Consortium is ready to fight the London Councils, which have approved a Bill calling for a total ban on the estimated 1.6 billion throwaway carrier bags used every year in the capital. Failing that, the Bill proposes a move to charge shoppers a levy. The tax would be used on environmental initiatives.

The cross-party London Local Authorities (Shopping Bag) Bill, which will be presented to Parliament next week by Westminster City Council, follows a six-week London-wide consultation. The study indicated 91.5% of 1,752 respondents want retailers to take action on carrier bags. As many as 58.2% support an outright ban, with 34.4% preferring a levy and 7.4% suggesting no action.

London is the first major city in the UK to consider a ban but scores of towns and villages across the UK are either trialling bag bans or considering them.

Earlier this year, 21 of the UK's biggest retailers - including the big four - joined forces as part of a Wrap-led commitment to reduce bag usage by 25% by 2008. Individual retailers have also made specific commitments, with Tesco, for example, awarding shoppers Green Clubcard points if they reuse bags. M&S meanwhile plans to introduce a 5p charge for carrier bags in the south west from next year following a successful trial in Northern Ireland.

"We do not need this legislation when retailers are already reducing bag usage. It is draconian and arrogant," says Peter Woodall from the Carrier Bag Consortium - a group of major UK carrier bag suppliers. "When I attended a London Assembly meeting last week on this issue we heard evidence from Tesco, M&S and Sainsbury's. They expected to meet or exceed the 2008 target."

"Retailers are bending over backwards to reduce carrier bag usage," adds Richard Dodd, head of media and campaigns at the BRC. "An outright ban would be a real inconvenience for shoppers and fails to take into account issues such as impulse purchases, protection of goods and shop security."

Despite spending more than £25m a year giving out free carrier bags to shoppers, Asda believes an outright ban is pointless without support from local councils. "We believe the focus should be on providing better recycling facilities," says a spokesman. "Local councils need to do more to ensure there is a consistent approach from postcode to postcode, so that regardless of where you live, you are able to recycle packaging and plastic bags more easily."

The multiples are unwilling to let go of plastic bags because they are a vital marketing tool, says Vicki Hird, Friends of the Earth's supermarket campaigner. However, an outright ban would not have the impact many fear, she argues.

"The supermarkets have a huge marketing benefit from their logo being seen everywhere," she says. "But if all retailers have to do the same, the competitive issue is lost so they should welcome an across-the-b0ard measure and start thinking creatively about new options to satisfy consumers and get their logo seen everywhere."

The Bill certainly has the momentum to make it through Parliament but the process could take up to two years. This week, prime minister Gordon Brown said he would back a ban and was planning to hold talks with retailers and the BRC to discuss possible action. "There is a lot of work to do but we are convinced we have done enough to get this through," says a London Councils spokesman.

The time is ripe for action, says Scottish LibDem MP Mike Pringle, who last year failed to get support for a Bill calling for a levy on plastic bags in Scotland but has now tabled a new motion with Scottish Parliament with a view to reintroducing the Bill. This time around, he already has the support of seven MSPs. "I am delighted this issue is finally getting the attention it deserves," he says.n

Tesco "Carrier bags are useful and rather than demonising them we will continue to find ways to reduce the number we issue while not penalising customers"

Sainsbury's "We encourage customers to reduce, recycle and reuse bags"

Asda "We're committed to reducing bag consumption by 25% by the end of 2008. However, banning plastic bags is not necessarily the best solution. The focus should be on better recycling facilities"