By Jeremy Lovell

LONDON, Feb 29 (Reuters) - British Prime Minister Gordon Brown on Friday warned retailers they had to start charging shoppers for the 13 billion plastic bags they currently get for free each year or the government would step in to force them.

Most bags end as landfill waste or being blown across the countryside, littering the landscape and harming wildlife. They can persist in the environment for centuries. Producing them involves petrochemicals and climate-damaging fossil fuels.

"I am convinced we need to act -- and the time to act is now," Brown wrote in the Daily Mail newspaper. "And I want to make clear that if government compulsion is needed to make the change, we will take the necessary steps."

"We do not take such steps lightly -- but the damage that single-use plastic bags inflict on the environment is such that strong action must be taken," he added.

International moves to curb the use of plastic bags are gathering momentum, with China and Australia announcing crackdowns last month. Ireland, Denmark and South Africa are among countries where customers must pay for bags.

Last year Brown struck a deal with British supermarkets to reduce the number of bags and increase their recycled content to cut the environmental impact by 25 percent.

But he said more needed to be done. "I have already made clear that over time we should aim to eliminate the single-use plastic bag altogether," he wrote.

The warning comes a day after retailer Marks & Spencer said it would start charging five pence (10 cents) for every single-use bag a food shopper took from next month, with the money going to improve parks and play areas across the country.

Critics accused Brown of tokenism, but the government says it is serious about getting people to change their lifestyles to protect the environment.

A climate change bill currently going through parliament and expected to become law within three months sets the government a legally-binding target of cutting emissions of carbon dioxide by 26-32 percent by 2020 and 60 percent by 2050.

Brown has said he wants a committee that will be set up by the bill to see whether raising the end target to 80 percent is necessary or feasible. Environmental groups say it is both.

(Editing by Mark Trevelyan)

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