Research from Oxford University showed the average man was 7.7kg (17lb) heavier in 2000 than he was in 1986.
Women have not fared much better, with an average weight gain of 5.4kg (12lb) between the mid-1980s and 2000.
While men have put on weight because of poor diet and a lack of exercise, women have piled on the pounds by simply eating too much.
The research, funded by the British Heart Foundation (BHF), analysed changes in food consumption and body weight over a long time period. It was published in the British Journal of Nutrition.
Eating extra calories was enough to make men put on almost 5kg in weight, but they piled on even more pounds because of a lack of exercise.
NHS figures for 2008 in England, the most recent available, show that 25 per cent of men were classed as obese, compared with only around 7 per cent in 1986/7.
Dr Peter Scarborough, who led the latest study, said: "There could be a number of reasons for the reduction in exercise.
"One partial explanation could be that men spend more of their working lives sitting at desks now – manual careers are less common than they used to be.
"We looked at how much food was available over time, accounting for food that's wasted or thrown away.
"It's clear people are eating more, and today we're seeing a continued increase in the amount of food available."
The authors said achieving a balance in future should include "addressing the obesogenicity of the food environment".
Tam Fry, from the National Obesity Forum, said: "It was figures like these that gave the UK ~ its first obesity wake-
up call in 2001.
"Sadly, the initiatives of the last 10 years have done nothing to improve the upward surge of the epidemic. There are no current plans on offer to ensure we don't hit the same rise again in this decade."