It has been estimated that 1m children will be obese in England by 2012.
The current generation of children faces a far higher risk of cancer later in life due to their unhealthy habits, a specialist has warned.
Professor Kathy Pritchard-Jones said western countries should prepare for an "explosion" of weight-related cancers.
The president of the European Society for Paediatric Oncology said governments should act now.
An estimated 300 million adults worldwide are currently clinically obese.
Professor Pritchard-Jones, based at the Institute for Cancer Research in Sutton, Surrey, made her comments to mark World Cancer Day.
She said that while children with poor lifestyles were no more likely to suffer from cancers in childhood, they were storing up trouble for later life.
"Childhood is the time when the habits of a lifetime are established. If you want healthy adults you have to start by making healthy children.
"If we don't do something about tackling how much exercise our young people take and how concerned they are about what they eat and their weight, we are going to have another explosion of cancers, to which unhealthy lifestyles will be a significant, contributory factor."
She said that the western world had just started to make an impact on smoking-related cancer, but this would be cancelled out by the rise in obesity related illness.
At present, poor diets, lack of exercise, and people being overweight or obese accounts for approximately a third of cancers in western countries, and approximately a fifth in developing countries.
The International Obesity Taskforce estimates that one in ten school age children is overweight, and about 30 to 45 million worldwide are said to be obese.
'Not well known'
Professor Pritchard-Jones called on families, health professionals, teachers and governments to do more.
"I think World Cancer Day should make governments and the world's policy makers take a look at what is happening in their own populations, and this may stimulate them into action."
Tam Fry, from the Child Growth Foundation, said there had been "unacceptable" delay on the part of the UK government.
He said: "This government has been putting off the day on which it is going to have to take on the food industry and say: 'Enough is enough'.
"The cancer campaigners are right in saying that a whole generation is now at risk of being affected."
Professor Alexander Eggermont, the president of the European cancer organisation ECCO, said that the rise in obesity-related cancers was not as well-known as some of the other health effects.
He said: "The focus on overweight and obesity is very important and timely. Making people aware of the dangers of smoking, as well as the dangers of unhealthy eating, should be part of all public awareness programmes and national cancer plans."
A spokesman for the Department of Health said: "We continue to work closely with the food industry on labelling, advertising and reducing salt and saturated fat.
"Ofcom reported in December that new rules on advertising have had an impact on reducing children's exposure to junk food advertising.
"The UK is leading the world when it comes to facing up to the problem and tackling obesity.
"We are aiming to change the way we all live our lives through Change4Life, which will help us all to change the way we eat, the way we exercise and the way we raise our children so we can prevent obesity and related diseases."