Parents are often unaware when their children are overweight or obese, a study says.
The role of parents in tackling obesity is essential, experts say
Researchers surveyed parents of 277 children and found only a quarter of them recognised when their offspring were overweight.
Where children were obese, a third of mothers and 57% of fathers thought they were "about right", the team at Derriford Hospital in Plymouth found.
Denial and desensitisation to excess weight were to blame for the lack of parental concern, the team said.
The report comes at a time when child obesity rates are soaring.
Among youngsters aged two to four, obesity almost doubled between 1989 and 1998 from 5% to 9%, according to World Health Organization figures.
Among those aged six to 15, obesity rates trebled from 5% to 16% between 1990 and 2001.
The study also said parents were less likely to recognise overweight boys than girls.
Only 27% of overweight or obese boys were classed as at least "a little overweight" compared with 54% of overweight girls.
Overall, 19% of the children, 52% of mothers and 72% of fathers were either overweight or obese.
Parents were also unaware of their own weight problems, with 40% of overweight mothers and 45% of overweight fathers saying their own weight was fine.
Report co-author Alison Jeffrey, a senior research nurse at the hospital's EarlyBird Research Centre, said acknowledging excess weight was "essential" to tackling the problem.
"The reasons for poor awareness might include denial, reluctance to admit a weight problem, or desensitisation to excess weight because being overweight has become normal."
She also said the study, published on the British Medical Journal website, showed obesity was a problem across all social groups.
Dr Ian Campbell, president of the National Obesity Forum, said he was surprised by the findings.
"Previous research had suggested parents were quite good a picking up on the problem.
"The pertinent point is that if parents do not identify that there is a problem, then children are less likely to get help."
A separate survey of 319 parents commissioned by Norwich Union Healthcare found more than 80% of parents intend to be more accountable for their child's weight.
Professor Sarah Stewart-Brown, from Warwick Medical School, added: "What happens in the family is extremely important.
"It is almost impossible for children to address this issue on their own.
"While children are responsible for what they put in their mouths, parents are responsible for what is put on their plates."
She said the traditional assumption that excess weight was just "puppy fat" was wrong.
However, she said people would become more aware of the dangers of obesity following the Public Health White Paper but it would take time.
The paper, published last week, proposed a range of measures to combat obesity, including a coding system for food and a ban on junk food TV advertising before 9pm.