Asda says a trial involving automatic plastic bag dispensers has cut carrier usage by 30% in the two stores it is running.

The units, placed at the end of checkouts, dispense a bag only every 30 seconds, which discourages overuse, according to the retailer.

"Customers are not able to grab a handful at a time so they concentrate on packing more into one bag," said Guy Mason, Asda public affairs manager.

But Mason, who was speaking at an All-Party Parliamentary Retail Industry Group meeting on environmental issues, said Asda did not support the idea of a plastic bag tax - mooted recently by local authorities in London. "We think the best way is to encourage customers to use fewer and look at making the bags thinner," he said.

"In Ireland [the plastic bag levy] appears to have been a success, but now it is emerging that people are buying more black bin liners because they had previously used carrier bags for rubbish."