As new forecasts show obesity rates in the UK could surge to 40% by 2030, experts say governments need to take tougher action to fight the "global epidemic".

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A new study published in the Lancet shows that 23 million people in the UK will be obese in 20 years' time. That would see obesity soar from 26% to up to 48% for men, and up to 43% for women.

'Governments failing'

Scientists say the research casts doubt on obesity prevention schemes and shows governments around the world are failing to intervene to protect their citizens' health.

The World Health Organisation's Timothy Armstrong told Channel 4 News there needs to be a radical shift in the way obesity is tackled.

"It's very much up to goverments to implement policies to prevent obesity, but all of us have a role to play in curbing what's become a global epidemic."

The question is how much tax would you have to put on a soft drink or an unhealthy food to change an individual's behaviour and stop them buying it? Timothy Armstrong, World Health Organisation, Head of Surveillance and Prevention unit

He said governments could implement a range of measures, including restrictions on junk food advertising, traffic light labelling on food and school-based education programmes. But he said governments also need to work across the sector to change the products that are on the market and ensure healthy foods are more affordable.

"There needs to be a dialogue with the private sector to reformulate products. And if companies aren't willing to adapt, then governments can regulate on issues."

Obesity in the UK is to surge by around 40% by 2030.

Does a 'fat tax' work?

Mr Armstrong said that while taxing unhealthy foods could help reduce obesity, this remains a complicated process.

"It's still in the very early stages so we don't yet know if they are effective.

"Many countries are looking at implementing taxes to encourage people to make healthier choices.

"But you have to ask whether others are using the taxes to try to increase internal revenue during a time of economic difficulty," he said.

Finland and Norway have already taken the step of imposing a "fat tax" while Hungary will introduce a tax in September on pre-packaged foods which are high in sugar, fat or salt.

But Mr Armstrong said that some companies have been willing to absorb the tax, effectively removing any financial incentive to steer away from junk food.

"Certainly we know that taxing tobacco products and alcohol is an extremely effective way of changing consumer behaviour. But those taxes are quite high.

"The question is how much tax would you have to put on a soft drink or an unhealthy food to change an individual's behaviour and stop them buying it?"

'Nanny state' fears

Professor Klim McPherson, a co-author of the Foresight Report on obesity in 2007, said politicians were hampered in their fight against obesity by their fear of the "nanny state".

"They don't want to be labelled with that particular insult, as they see it.

"I think they do 'get it' but they don't know what to do about it, and they don't think it's their essential responsibility," he said.

The cost of our expanding waistlines

Worldwide, obesity has more than doubled since 1980.

If obesity across the UK goes up by the predicted 40 per cent by 2030, it will cost the NHS an extra £2bn a year - the equivalent of 2% of health spending.

The American health system would face an even greater burden, with predictions that obesity rates there would rise from one in three to about one in two.

Clinical obesity is defined as a body mass index (BMI) of 30 or higher. A healthy or normal BMI is between 18.5 and 24.9 while a BMI of 25 to 29.9 falls in the overweight range.