Experts believe that as soon as children leave behind the manic "terrible twos" phase they adopt a couch potato lifestyle that poses a long-term threat to their health.
The new research, reported in the British Medical Journal, is being touted as the first hard evidence of the scale of the problem.
Researchers from Glasgow and Bristol studied 1,000 children born in Bristol in 1991 and 1992.
They measured and weighed each child each year, and determined whether they were overweight by calculating their body mass index (BMI).
They found 16% were overweight at the age of two, but that by the age of four the proportion had risen to 20%.
Six per cent of the two-year-olds and eight per cent of the four-year-olds were defined as obese.
Obesity numbers rise
The findings show that the proportion of children who are overweight and who are obese has increased since the 1980s.
Physiologist Dr John Reilly, of Glasgow University's department of human nutrition, said: "This is probably an underestimate of the problem. Body mass index does not identify every child who has a high level of body fat."
Other studies show today's youngsters are eating less than in the past, but are watching more television at an early age.
Dr Reilly said: "What we suspect is happening is that pre-school children have been replacing natural active behaviour with inactive behaviour like watching TV.
"Children have been eating less, but are watching more TV - which includes watching videos, Playstations or PC screens.
"The percentage increases do not sound a lot - but if you imagine a large school or nursery intake of 1,000 youngsters, you would expect 50 children to be obese, but we are finding around 80."
Background Briefings
Medical notes
Breast-feeding 'prevents obesity'
(16 Jul 99 | Health)
Obesity threatens ill health epidemic
(03 Jun 99 | Health)
Task force takes on obesity
(27 May 99 | Health)
Obesity rise 'founded on denial'
(19 Apr 99 | Health)
Fat hope for an obesity cure
(13 Jan 99 | Health)
British Medical Journal
Obesity. com
Glasgow University
The BBC is not responsible for the content of external internet sites.
Links to other Health stories