Claims over better recycling rates than competitors were adjusted for growth in sales.
Tesco has been accused of releasing figures which could give the misleading impression that it met an industry target to halve its use of plastic bags.
Seven supermarket chains - Tesco, Asda, Sainsbury's, Co-op, Marks & Spencer, Somerfield and Waitrose - signed an agreement last year committing them to reducing the number of bags by 50 per cent over the three years to May 2009.
This month the supermarkets collectively reported, via figures released through recycling watchdog Waste and Resources Action Programme (Wrap), that they had fallen just short of the target, collectively achieving a 48 per cent reduction from 870 million bags in May 2006 to 450 million in May 2009.
Tesco issued a statement on the same day claiming it had met the target. " Our customers are now using more than 50 per cent fewer carrier bags than they did before," the statement said.
However, reports in The Times yesterday suggested that the company had overstated the reduction in plastic bag use.
A spokeswoman for the supermarket admitted that its figure had been adjusted to account for growth in sales and did not reflect the actual level of reduction.
"In working out how many carrier bags our customers are saving, it is misleading simply to compare the overall number of bags issued by a single retailer now with those issued in 2006," she said. "More customers now shop with Tesco than did in 2006, and on average each is using 50 per cent fewer bags than they did then. This is the most meaningful number. We believe this is a huge achievement in three years."
A Wrap spokesman said its agreement was with the retail sector as a whole and measured an absolute reduction in the number of single-use carrier bags - without taking any sales growth into account.
"The agreement is voluntary and data has been submitted on a confidential basis, so Wrap doesn't release or comment on data from individual retailers," said the spokesman. "It is up to retailers if they want to release their data publicly."
However, environment secretary Hilary Benn told The Times that he would like to see Tesco be more open about any reduction in plastic bag use. "I would encourage stores to let customers know what progress they are making," he said. "I would encourage that information to be made available."
Wrap figures show that supermarkets issue more than 4,700 tonnes of plastic bags every month and most are used only once.
The news comes as green groups called for the introduction of an effective watchdog for supermarkets.
Friends of the Earth's food campaigner Helen Rimmer said that there was an urgent need for a dedicated supermarket watchdog with the power to tackle anti-competitive behaviour and ensure environmental best practices are followed.
"The government must stand up to the big retailers and set up an independent watchdog to put an end to the supermarkets' bullying behaviour and secure a fairer deal for shoppers and farmers alike," she said.
The call follows a failure by the Competition Commission to get an agreement from government over the establishment of a regulator, despite finding evidence that supermarkets were guilty of abusing their buyer power.
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