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A 20% tax on sugary drinks in the UK would prevent 3.7 million people becoming obese over the next decade, a report predicts.

Cancer Research UK and the UK Health Forum worked out the likely impact of the tax on eating habits and, ultimately, the nation's waistlines.

Their report said such a tax would also save the NHS £10m a year by 2025.

The government is considering the measure, but soft drinks companies say other options would be more effective.

People get between 12% and 15% of their energy from sugar, but official recommendations say it should be less than 5%.

Fewer calories

The ripple effect of a small tax on sugary drinks is enormous Alison Cox, Cancer Research UK,

The statisticians modelled the impact of the tax and predicted a 16% reduction in the number of cans of pop consumed.

Their figures were then adjusted to account for the food and drink people might turn to instead.

They concluded a tax would lead to people consuming on average 15 fewer calories per day.

While the difference sounds tiny, the model predicts a large impact on waistlines.

Currently, 29% of people are obese and trends suggest that figure will reach 34% in 2025.

Rather than reverse the obesity epidemic, the forecast predicts the tax would lead to obesity rates levelling off at around 29% - preventing 3.7 million people from becoming obese.

Alison Cox, from Cancer Research UK, said: "The ripple effect of a small tax on sugary drinks is enormous.

"These numbers make it clear why we need to act now before obesity becomes an even greater problem."

And Jane Landon, from the UK Health Forum, argued: "Countries which have introduced a tax on sugary drinks have not only reduced consumption, they have raised much-needed revenues for public health measures."

The report has come out as the government is preparing its obesity strategy, which is due in the next few weeks.

Prime Minister David Cameron and Health Secretary Jeremy Hunt have indicated all options are on the table, including a sugar tax.

A report by Public Health England said a sugary drinks tax would cut sugar consumption "at least in the short term".

And TV chef Jamie Oliver has campaigned for the tax saying it was the "single most important" change that could be made.

The director general of the British Soft Drinks Association, Gavin Partington, said: "The hypothetical claims made in this modelling study run contrary to real-world evidence.

"In fact, the soft drinks tax in Mexico has reduced average calorie intake by six calories per person per day, with no evidence that it has reduced levels of obesity."

He added that other options such as reducing portion sizes or changing ingredients would be more effective.

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The sugar problem

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There has been growing concern about the damaging impact of sugar on health - from the state of people's teeth to type-2 diabetes and obesity

Sugar has been dubbed "empty calories" because it has no nutritional benefit

Government advisers recommend no more than 5% of daily calories should come from sugar

That is about 1oz (25g; six or seven teaspoons) for an adult of normal weight every day. For children, it is slightly less

The limits apply to all sugars added to food, as well as sugar naturally present in syrups and honey

To put this in context, a typical can of fizzy drink contains about nine teaspoons of sugar

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