Around a quarter of British men are "in denial" about their waist measurement, a Cancer Research UK survey suggests.
Men are being urged to reduce their waistlines
Official statistics show that 65% of adult men are overweight or obese, but the survey of over 2,000 men found only 40% thought they were in these groups.
Most of the men questioned were unaware being overweight or obese, or inactive, increases the risk of cancer.
Experts urged overweight men to become more active and eat a balanced diet rich in fruit and vegetables.
The survey was released to launch Cancer Research UK's Man Alive Campaign.
Around 65% of those questioned had no idea that being overweight or obese increased their chance of developing cancer, with 75% not aware that being physically inactive also increased their risk.
The survey also found just 22% of those questioned eat five portions of fruit and vegetables per day, and two-thirds fail to meet the recommended target of 30 minutes of moderate exercise five times a week.
Cancer Research UK experts said men were not heeding health warnings to maintain a healthy body weight.
They warned obesity could be responsible for as many as 40,005 cases of cancer in UK men every year.
Recently published research suggested obesity increases the risk of bowel cancer by 60%.
Being obese also doubles the risk of being diagnosed with kidney, oesophageal and stomach cancer, as well as being a risk factor for bladder cancer.
Good diet and regular exercise are two factors that can help in reducing the risk of developing cancer, they said.
Professor Jane Wardle, the director of Cancer Research UK's Health Behaviour Unit, said: "In the space of 10 years, the rate of obesity for British men has increased two thirds.
"Women are overweight too but our research shows that more of them know they have a problem and that's the first step in putting it right."
She added: "Men need to be made aware of the problem, know how to fix the problem and be given the support to succeed.
"Only then will they reduce their risk of cancer."
'Wake up call'
Dr Lesley Walker, director of information at Cancer Research UK, said: "It's vital that men understand how they can reduce their risk of cancer.
"The best present that anyone can give themselves is to stop smoking, it is by far the most important step in reducing cancer risk."
But she added: "In men who don't smoke, obesity is one of the biggest known causes of preventable cancer.
"With rates of obesity for UK men growing faster than anywhere else in Europe, we are sounding a wake up call to all men and their partners to become more active, eat healthier diets and make sensible lifestyle choices.
"To lose those spare tyres, be more active and eat a balanced diet that is rich in fruit and veg and low in sugar and fat."