Customers at the UK's leading supermarkets used 43% fewer carrier bags in 2009-10 than they did in 2006, when figures were first recorded, with 6.1bn single-use bags used in 2009-10 against 10.7bn four years earlier.

According to the Waste & Resources Action Programme (Wrap), the amount of material used to make carrier bags has reduced by 39,700 tonnes per year in the past four years, but figures for May 2010 alone indicate that we may be seeing a return to greater usage.

Data collected from Asda, Cooperative Group, M&S, Sainsbury's, Somerfield (now owned by the Cooperative Group), Tesco and Waitrose shows that during the past year, total bag usage in the UK fell by 0.7bn, or 10%. Single-use carrier bags fell by 9% in the UK in total during that time, with Wales experiencing a 15% drop and Scotland a 13% drop, followed by Northern Ireland with a 9% fall and England with an 8% drop.

A spokesman for Marks & Spencer said: "We started charging for food carrier bags in all stores in May 2008. Last year we used 89m single-use carrier bags in our food business – over 80% less than 2006/07. This has been achieved by encouraging customers to re-use, use bags for life and charging 5p per carrier bag. Across our stores, carrier bag use is down by 64% since 2006."

In 2008, the UK government, along with the British Retail Consortium (BRC) and the seven supermarkets agreed to a voluntary approach to cut the number of single-use bags given to customers by 50% by spring 2009. In May last year, Wrap reported that retailers had cut the number of single-use bags by 48%, falling short of the target. Although the formal agreement ended last year, this year retailers volunteered to share their figures.

But figures for the month of May suggest that since the voluntary agreement ended, the supermarkets' performance has dipped. In May, the number of single-use bags increased slightly from May 2009, with 45% fewer bags used this May than in May 2006, compared to 48% fewer in 2009.

It means UK consumers used 7.7 bags per person during the month. In Wales, that figure was 8.9 bags per person and in Scotland, 8.3 bags. Shoppers in England used 7.6 bags, while the figure in Northern Ireland was 8.1.

A spokesman for waste campaigners Waste Watch said: "If we are seeing a return to more carrier bag usage and retailers are not able to reduce numbers, we would like to see a bag tax introduced. That said, carrier bags are totemic and not the biggest waste issue we face. There are many more single-use products out there that we would like to see eliminated such as disposable picnic plates and cutlery, and disposable razors. We need to get into the habit of reusing things."

Stephen Robertson, BRC director general, argued that the headline reduction figures showed that customers were permanently adopting the habit of re-using their bags. "Retailers are working hard on a range of other environmental measures, such as reducing food waste, reducing and redesigning packaging, as well as providing customers with recycling information through the on-pack recycling label."