LONDON (NYTIMES) - When the British government introduced a 5-pence levy on plastic bags four years ago, it encouraged shoppers to help reduce environmental damage by bringing their own reusable "bags for life".

But the bags - which are sturdier than traditional single-use plastic bags - have instead become a significant factor in the country's largest supermarkets' "plastic footprint", according to a report published on Thursday (Nov 28) by Greenpeace and Britain's Environment Investigation Agency.

This year, the 10 companies representing most of Britain's grocery retail market have sold more than 1.5 billion "bags for life", the report found - which amounts to 54 bags per household. That was on top of the 959 million "bags for life" sold in the country's main supermarkets last year.

"We have replaced one problem with another," said Fiona Nicholls, a Greenpeace UK campaigner who is one of the report's authors. "Bags for life have become bags for a week."

One company alone, the frozen-food chain Iceland, reported a ten-fold increase in sales of "bags for life" this year, the report found - 34 million bags, up from 3.5 million last year.

In promoting the sturdier bags in 2015, the government said, "Typically, you pay for these once and can return them for a free replacement when they wear out."

Yet four years later, the campaigners' report, titled "Checking Out on Plastic II", found that the "bags for life" sold by the largest supermarkets this year amounted to nearly 50,000 tons of plastic, in addition to more than 3,330 tons of plastic from their single-use bags this year.

Overall, total plastic packaging in Britain's main supermarkets amounted to 995,000 tons last year, the report found, although some supermarkets were experimenting with selling more loose produce rather than relying largely on fruit and vegetables wrapped in plastic.

After its introduction, the plastic bag levy was credited with a more than 80 per cent reduction in the number of bags given out by the largest retailers. The extent to which increased "bag for life" sales have countered this effect was unclear.

Shoppers in Britain have widely debated plastic pollution for years, with public awareness significantly increasing after millions watched the hit BBC documentary series "Blue Planet II" in 2017.

Its renowned narrator, David Attenborough, later urged people to reduce their plastic footprint, as government officials said they had been "haunted" by the documentary's images of the damage that plastic has done to the world's oceans.

Nearly 9 million tons of plastic end up in the ocean every year, according to Ocean Conservancy, a US-based nonprofit environmental group.

In 2018, the country's largest supermarket chains, including Tesco, Sainsbury's and Waitrose, joined an industrywide initiative to "tackle the scourge of plastic waste." The initiative, known as UK Plastics Pact, included a pledge to replace traditional plastic packaging with reusable, recyclable or compostable material, although campaign groups say that such a move is unlikely to reduce the companies' plastic footprint.

Yet despite the British public's increased knowledge of environmental issues around plastic, Nicholls said that supermarkets and retail companies were performing poorly in lessening their plastic footprint.

"After all this public awareness, we'd expect plastic consumption to drop in supermarkets, but it's actually increasing," she said.

To encourage shoppers to reuse bags, the report urges British supermarkets to raise the price of "bags for life" - which currently sell for 20 pence (S$0.35) in Sainsbury's - to at least 70 pence, "or ideally to remove them altogether".

Sales of "bags for life" fell 90 per cent in neighbouring Ireland when supermarkets significantly raised the price of the bags, according to the report.

That kind of nudge aims to raise the likelihood that people will take their own bags when visiting a supermarket.

"When we go shopping," Nicholls said, "we should remember our bags like we remember our phones."