LONDON (Reuters) - Britain on Thursday said it would tax companies which sell sugary soft drinks and invest that money in health programs for school children, part of a long-awaited strategy to curb childhood obesity that critics say is too weak.

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Drinks companies were also angered by the plan which urges industry to cut sugar in products aimed at children, saying nearly a third of those aged 2 to 15 are already overweight or obese.

In a statement announcing details of the strategy, which has been in the works for several years, junior finance minister Jane Ellison said obesity was costing Britain’s National Health Service (NHS) billions of pounds every year.

Campaigners and health experts, however, said the plan was weak.

Graham MacGregor, a professor of cardiovascular medicine and chairman of the Action on Sugar campaign group, said it was “an insulting response” to Britain’s obesity and diabetes crisis which “will bankrupt the NHS unless something radical is done.”

In opting for a sugar tax, Britain joins Belgium, France, Hungary and Mexico, all of which have imposed some form of tax on drinks with added sugar. Scandinavian countries have levied similar taxes for many years.

Britain’s plans will see a levy applied to drinks with a total sugar content above 5 grams per 100 ml (about a teaspoon per 3.38 fluid ounces for US measures), with a higher band for even more sugary drinks.

The government’s health department says sugary drinks are the single biggest source of sugar for children, and a child can have more than their recommended daily intake just by drinking a can of cola which contains nine teaspoons of sugar.

It wants the industry to work towards a 20 percent cut in products popular with children, with 5 percent in the first year. Progress would be reviewed every six months by the government’s health agency, Public Health England.

But Gavin Partington, Director General of the British Soft Drinks Association, said the levy was a “punitive tax” that would “cause thousands of job losses and yet fail to have a meaningful impact on levels of obesity.”

Sara Petersson, a nutrition analyst at Euromonitor International, said the focus on sugar may detract from other crucial factors in obesity.

“It is becoming abundantly clear that replacing a critical ingredient of a product, or single nutrient in a diet, is neither an easy process for food companies nor a successful obesity strategy,” she said.

The program the government intends to launch with funds raised from the sugar levy will focus on promoting healthy diets and physical activity in schoolchildren, Public Health Minister Nicola Blackwood said.

She said primary schools would be asked to help pupils get at least 60 minutes of moderate to vigorous exercise every day. At least 30 minutes of this should be during school time, she said.