Nearly two-thirds of overweight people recognise the perils of their heftiness but fail to do anything about it, Cancer Research UK said.
The number of cancers linked to weight, such as kidney and womb cancers, are increasing rapidly, the charity said.
Lack of willpower is the biggest barrier to losing weight, with 68pc of women and 60pc of men of the 2,011 people surveyed citing it as the main reason which stops them.
Charity chief executive Dr Harpal Kumar said: “Unless we tackle the obesity epidemic in the UK, we risk cancer cases soaring.
“We understand that it can be extremely hard for people to maintain a healthy weight but keeping those extra pounds at bay could ultimately save your life.”
Professor Jane Wardle, from Cancer Research UK’s Health Behaviour Research Centre at University College London, said: “Our report shows that even though overweight people would like to lose weight and are aware of the cancer risk, they feel lack of willpower is a major barrier to shedding the pounds.
“We know that the modern-day environment makes it very hard for people to lose weight, especially when they are bombarded by advertising and easily tempted by cheap, ready-made meals and fast food, instead of a balanced diet with plenty of fresh fruit and vegetables.
“But for both men and women, being overweight is – after smoking – the most important risk factor for cancer.
“What many people don’t realise is that extra fat around the middle, their muffin top, is surprisingly active, releasing hormones and other chemicals that can make cells in the body divide far more often than usual, which can increase the risk of cancer.”