Prime Minister Gordon Brown has today issued a stark warning to retailers: cut plastic bag use or the government will use legislation to force you to do so.

Writing in the Daily Mail in response to the paper's high-profile Banish the Bag campaign, Brown said he had already set the target of eliminating single-use plastic bags and that "if government compulsion is needed to make that change, we will take the necessary steps".

The comments will be interpreted as a thinly veiled threat to retailers to stick to their recent commitment to cut the environmental impact of plastic bags by 25 per cent by the end of the year or face legislation demanding they charge customers for plastic bags.

Brown said the government did not take the idea of introducing new laws on plastic bags lightly, but added that, "the damage that single-use plastic bags inflict on the environment is such that strong action must be taken.

"The important thing is to come up with a scheme that will be of most benefit, most quickly – and ideally at the same time secure funds for organisations that help protect our environment," he added.

The British Retail Consortium (BRC) said that legislation would not be necessary to cut plastic bag use, insisting retailers were on track to meet the target agreed last November to cut bags' environmental impact by 25 per cent.

"The voluntary schemes, such as free bags for life, bags made from biodegradable materials and incentives for not using bags, are working really well and we do not feel there is the need for further legislation," said a spokesman for the BRC. "The other advantage of this voluntary approach is that each retailer knows which scheme will best suit its customers."

In an embarrassing development for the government, The Guardian newspaper today reported that government departments distributed almost one million branded plastic bags last year.

The figures emerged in answers to a series of parliamentary questions by the Conservatives. Some departments supplied details for the past two years, allowing the paper to work out the total figure for 2007.