Britain’s biggest supermarket chain has published misleading figures giving the impression that it had met an industry target to halve the use of plastic bags.

The Times has learnt that Tesco, which claims to be one of the greenest retailers, missed the target and tried to conceal its actual performance.

Hilary Benn, the Environment Secretary, called on Tesco and other supermarkets to be more honest with customers about how many plastic bags they were issuing. He urged them to publish individual figures so that shoppers could compare them.

The Government has threatened to ban free carriers unless supermarkets make progress in cutting the number through voluntary agreements.

Most plastic bags end up in landfill sites where they can take up to 1,000 years to decompose. Supermarkets issue more than 4,700 tonnes of plastic bags every month and most are used only once.

Seven supermarket chains — Tesco, Asda, Sainsbury’s, Co-op, Marks & Spencer, Somerfield and Waitrose — signed an agreement last year committing them to cutting the number of bags by 50 per cent over the three years to May 2009. This month they reported, via the Waste and Resources Action Programme (Wrap), the independent waste watchdog, that they had not met 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 saying: “Our customers are now using more than 50 per cent fewer carrier bags than they did before.” However, after being questioned by The Times, Tesco admitted that its figure had been calculated in a different way to the one issued by Wrap.

It said that its figure had been adjusted to account for growth in sales and did not reflect the chain’s actual performance. A senior source at Tesco said that the actual reduction had been significantly less than 50 per cent but the company was concerned about negative publicity.

A Tesco spokeswoman said that it would not be publishing the actual number of bags used and would not be revealing by how much that number had declined. She said: “We gave Wrap the absolute number. I don’t think it would be appropriate for me to give you that figure.”

Tesco continues to defend its policy, claiming that measures such as giving Clubcard reward points for reusing bags is more effective. It said on its website: “We believe that climate change will only be tackled successfully if people are encouraged to change their behaviour willingly.”

Speaking to The Times, Mr Benn said: “I would encourage stores to let customers know what progress they are making. I would encourage that information to be made available.”

Mr Benn said that he was not yet persuaded of the case for a ban but said that the Government needed to set the industry a clearer target. “We are going to review progress in summer 2010. We will see how we are going in a year’s time.”

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