Almost 700,000 more people could develop cancer in the next 20 years due to being overweight or obese, Cancer Research UK has warned.
The charity - which is for the first time calling for a tax on sugary drinks, backed by celebrity chef Jamie Oliver - said immediate action is needed to tackle Britain's "alarming" obesity levels.
Ten types of cancer are linked to obesity, which can also lead to Type 2 diabetes, heart disease, stroke and a range of other health problems.
In a new report, Cancer Research UK and the UK Health Forum warn that current trends mean almost three in four adults will be overweight or obese by 2035.
It also predicts that more people will be obese than overweight by 2030, and the rise in obesity will cost the NHS an extra £2.5 billion a year by 2035.
But just a 1% drop in the number of people going from being overweight or obese to a healthy weight every year could prevent more than 64,000 cancer cases over the next 20 years and save the NHS £300 million in 2035 alone.
Cancer Research UK is calling for a 9pm watershed ban on TV advertising of junk food as well as a 20p per litre tax on sugary drinks.
It says this is needed to cut the amount of sugar consumed, including by children and teenagers.
Oliver said: "Being overweight or obese can cause many diseases including some cancers.
"But I believe we can prevent the rising trend in obesity in the UK and across the world. Food education is key here. If governments take children's health more seriously and use education to inspire them, we could have a huge impact on their health and wellbeing.
"We're raising a generation of children in a society where junk food is cheap, widely advertised, and packed full of sugar so it's difficult to teach them how to make healthy choices.
"We need to give these kids a chance to be healthier adults - starting with a tax on sugary drinks to tackle obesity and diet-related disease in young people."
The Government is expected to publish its obesity strategy in the next month or so, but has so far resisted calls for a sugar tax on drinks or food.
Prime Minister David Cameron is known to be against the move, with a Downing Street spokesman saying in October that he believed there were "more effective ways of tackling" obesity.
A controversial report from Public Health England has said a sugar tax of 10% to 20% could work alongside other strict controls, including a ban on advertising unhealthy foods during "family" TV programming such as The X Factor.
Alison Cox, director of cancer prevention at Cancer Research UK, said: "Obesity will be a huge burden to society and the NHS in the near future. We must act now to combat this threat and we need the Government to restrict the marketing of sugary food to children.
"Kids are bombarded with advertisements for unhealthy food. It's vital the Government restricts this kind of advertising if we are to give our children the chance for better and healthier lives."
Paul Lincoln, chief executive officer at the UK Health Forum, said: "The Government's planned childhood obesity strategy is a golden opportunity to tackle the availability, affordability and promotion of unhealthy foods that is driving the current crisis in children's diets."
Overweight is defined as having a body mass index (BMI) of between 25 and 29.9. Obese is defined as having a BMI of 30 and over.
The 10 types of cancer linked to obesity are of the bowel, breast in older women, gallbladder, liver, kidney, womb, pancreatic, oesophageal, and aggressive forms of ovarian and prostate cancer.
The report comes as research published in the British Medical Journal (BMJ) shows that a sugary drinks tax in Mexico has led to a 12% reduction in sales and a 4% increase in the purchase of untaxed drinks one year after it came in.
From January 1 2014, Mexico implemented an excise tax of 1 peso per litre on sugar-sweetened beverages.
Researchers in the US and Mexico examined data from more than 6,200 Mexican households across 53 large cities, and used modelling to come up with their results.
They found a rise in untaxed drinks sales, mainly due to more purchases of bottled plain water.