Obesity is set to become “the new normal” within two decades according to new forecasts warning of soaring cases of cancer caused by excess weight.

The figures from Cancer Research UK show a 45 per cent rise in the number of cases of cancer caused by excess weight in just two decades.

The charity - which is for the first time calling for a tax on sugary drinks - said urgent action is needed to tackle Britain`s "alarming" obesity levels.

On current trends, almost three in four adults will be overweight or obese by 2035, their study shows.

n total, 39 per cent of the population would be obese – dwarfing the 33 per cent who will be overweight and the 28 per cent forecast to be of healthy weight or less.

Meanwhile the number of cases of cancer caused by excess weight will rise from 26,600 cases a year to 38,500 by 2035, with almost 700,000 more cases of cancer due to excess weight during the two decades.

Health charities last night called for radical changes to the typical British lifestyle - which has already pushed obesity levels from just 3 per cent in the 1970s, to 25 per cent today.

The report, published as the Government draws up a national strategy on childhood obesity, also calls for a 9pm watershed ban on TV advertising of junk food.

Separate research published in the Lancet Diabetes and Endocrinology says reducing the sugar content of soft drinks by 40 per cent could prevent 300,000 cases of diabetes in 20 years.

Researchers at Queen Mary University, London, said targets to reduce sugar content, in the same way salt had been tackled, could mean one million fewer people became obese.

Health campaigners last night called for mandatory targets, warning that Britain has become “a nation hooked on the sweet stuff”. Supermarkets have said they would support such changes.

Britain’s obesity levels are already among the worst in Europe, second only to Hungary.

Ten types of cancer are linked to excess weight which can also lead to Type 2 diabetes, heart disease, stroke and a range of other health problems.

The Cancer Research UK forecasts suggest that in just two decades, excess weight will result in approximately 440,000 avoidable cases of obesity-related diseases per year - including diabetes, heart disease and stroke.

Alison Cox, the charity’s director of cancer prevention said action was needed to stop children being “bombarded” with advertisements and marketing.

"Obesity will be a huge burden to society and the NHS in the near future. We must act now to combat this threat,” she said.

The calls for a sugar tax follow those from Public Health England, more than 20 royal medical colleges and public health groups, and from celebrity chef Jamie Oliver.

The Government is expected to publish a strategy to combat childhood obesity over the next month, but has so far resisted mounting pressure for a sugar tax.

Prime Minister David Cameron is known to be against the move, with the Government thought to be more enthusiastic about introducing restrictions on advertising and marketing of junk foods.

Despite that, there have been indications that some ministers are relaxing their stance towards a sugar tax, according to reports in The Times newspaper.

Senior ministers are said to be studying the proposals after evidence emerged proving a link between such a move and reduced levels of obesity.

A 10 per cent sugar tax on fizzy drinks in Mexico has cut sales by 12 per cent, in the first year it was introduced, according to research published in the British Medical Journal.

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