More than 12m adults and one million children will be obese by 2010 if no action is taken, a report by the Department of Health is predicting.
The Health Survey for England also warns 19% of boys and 22% of girls aged two to 15 will be obese.
The figures would mean the government would fail to meet its target to halt the rise in childhood obesity.
The report also says having two obese parents means children have five times the risk of being obese themselves.
One in four children in households where both parents are obese is obese themselves, compared to one in 20 in households where both parents are a healthy weight.
In households where one parent is obese, one in eight children is also dangerously overweight.
The report warns that, based on current trends, 33% of men and 28% of women will be obese by 2010.
The government says it is the "most accurate estimate so far" of future obesity rates.
The data is published just days after a "minister for fitness" was appointed.
Caroline Flint will be working across all government departments to develop a new fitness strategy for England.
'Food for thought'
Health Secretary Patricia Hewitt said: "With children heading back to school in September, these statistics should give parents food for thought on how to make their kids' lifestyles healthier.
"We are intervening and helping to make a difference, but we want today's figures to act as a stark reminder of the problem we and our children will face if we don't act now and start making healthier lifestyle choices."
She told BBC Radio 4's Today programme: "The government's got a responsibility to make it easier for people to make healthy choices for themselves.
"But at the end of the day, it's up to each of us to decide what we eat, what we drink, how much exercise we take and how we bring our children up."
HOW TO STAY HEALTHY
Eat regular, balanced meals
Eat at least five portions of fruit and vegetables a day
Avoid foods that are very high in sugar and/or fat
Eat less than 6g of salt/day
If you drink alcohol, drink it in moderation
For adults: 30 minutes moderate activity five times a week
For children: an hour a day of moderate activity
The Department of Health is backing a scheme where GPs give obese patients diet and exercise plans. A pilot scheme has been so successful that 10 other areas are now interested in introducing it.
Dr Susan Jebb, Medical Research Council nutritionist, said the government's bid to halt the rise in childhood obesity by the end of the decade had always been an "unrealistic target" because of the length of time it took to address the issue.
She said it could take many decades to reverse the trend.
"People are not thin one day and fat the next, or vice versa."
Junk food ads
But Dr Jebb said there were things people could do now.
She said parents needed to focus on making healthy diet and activity choices for their children.
"Think about the food you bring into the house, and if you're treating children, take them swimming or to the park and not to the cinema."
But she said the food industry and the government also had roles to play in helping people make healthy choices.
"People don't choose to be obese. Everyone knows what to do, but when they get into a supermarket, they don't put that into action."
Shadow health secretary Andrew Lansley said: "The government needs to improve on its woeful record or the health of the nation will continue to suffer.
Sandra Gidley, Liberal Democrat health spokeswoman, said: "This week's rebranding of Caroline Flint as fitness minister was a gimmick intended to shift attention away from the government's failure to tackle the time bomb of obesity in this country."