Marks & Spencer believes that charging customers 5p for a plastic bag has cut usage by 80 per cent in spite of warnings that the move could deter customers.

The food and fashion retailer, which won plaudits from environmental groups when it introduced the scheme, will say today that customers have used 70 million fewer bags in the ten weeks that the charge has been in place. M&S aims to cut carrier bag usage by a third and to send no waste to landfill by 2012. It has sold ten million bags, donating the 1.85p profit on each to Groundwork, an environmental charity.

Campaigners said that M&S was moving in the right direction but analysts said that it risked alienating customers who might balk at the extra cost when they reached the checkout.

M&S, which issued a surprise profits warning at the beginning of the month, is under huge pressure to maintain sales momentum. Sir Stuart Rose, the executive chairman, raised the ire of investors when he was elevated from chief executive.

Neil Saunders, consulting director at Verdict Research, the retail research firm, said that it was a very brave move. “From what we have heard, sales have been reasonable but growth has not been as good as last year,” he said.

Keith Bowman, equity analyst at Hargreaves Lansdown, the stockbroker, said: “The other supermarket players have all introduced schemes to try to cut bag usage through their loyalty cards. M&S haven't given themselves the option of incentivising their customers with points. That penalises customers rather than rewards them.”

An M&S spokesman said: “We are a retailer. If this was hurting us, would we still be doing it?”

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