The majority of parents are no longer able to tell whether their children are overweight, a ground-breaking study of how British families perceive body size will reveal this week.

Even when their children are clinically obese, at least one-third of parents - particularly fathers - believe they are the normal weight for their age. Research to be presented at a conference shows they no longer recognise a healthy body shape for a child.

The study has alarming implications for Government plans to fight fatness, the country's largest public health epidemic. If families cannot spot obesity in their own children they are highly unlikely to follow much of the health advice being issued.

Researcher Alison Jeffery, who carried out the study, said: 'It looks like the health messages are falling on deaf ears. We'll be pouring money down the drain on this unless parents become more aware of what constitutes a healthy weight.'

As the average weight of British children soars, traditional body images have become so confused that some parents are complaining to GPs that their children are underweight when they are actually the right weight.

Jeffery, a researcher at the Peninsula Medical School of Exeter and Plymouth Universities, studied 300 children and adults, asking each parent to describe their own weight and that of their seven-year-old child.

The children were shown a chart and asked to describe which shape fitted them. The parents were shown it and asked which shape most corresponded with that of their child.

Just 25 per cent of adults with overweight children recognised the problem. None of the fathers identified their sons as overweight, even when they were. More worryingly, 33 per cent of the mothers and 57 per cent of the fathers described obese children as normal.

In the study, part a of long-term investigation into how diabetes may develop at an early age, seven per cent of the boys were overweight and 8 per cent obese. The figures for the girls were worse: 10 per cent and 13 per cent respectively.

Jeffery found no link between the parents' fat awareness and their class, education or income. Thinner parents were no better at identifying obesity than the fatter ones, contradicting the argument that overweight adults are more likely to see their children as thinner than they really are.

However, in a sign that girls face more discrimination throughout life about their size, Jeffery found that both mothers and fathers were quicker to recognise overweight in daughters than sons.

Two-thirds of the children were unaware of their weight but most - particularly girls - tended to underestimate their weight. The parents were not very accurate either. A quarter of the mothers and more than half of the fathers who were overweight thought they were 'about right'.

Jeffery said: 'People can't tell any more what is overweight. It's difficult to categorise children when they are slightly overweight, but what is worrying is that the parents didn't recognise serious weight gain in their own children.

'You might argue that it's not good to stigmatise a seven year old by identifying them as overweight, but there are much greater health risks that they face. Some of them are already becoming insulin-resistant, and a young girl who grows into an obese teenager will face all sorts of psychological problems such as teasing and bullying.'

Within a single generation, it seems the nation is beginning to see an overweight body as the normal size. Obesity in children aged between six and 15 has trebled in 11 years, and if current trends continue at least a third of adults, a fifth of boys and a third of girls will be obese by 2020.

Type 2 diabetes is becoming one of the greatest concerns because its prevalence is increasing rapidly in the UK, fuelled by poor food and lack of activity. Jeffery's study will be presented at an annual diabetes conference in Birmingham, where experts will hear that four out of five people with this diagnosis are overweight. Carrying extra weight, particularly around the waist, causes the natural insulin that the body produces to work less effectively.

Benet Middleton, chief executive of the charity Diabetes UK, said: 'People underestimating their body mass category and beginning to see being overweight as the norm is very worrying.

'Being overweight can lead to numerous health problems including Type 2 diabetes. Obesity needs to be tackled now.'

· Jeffery's work can be seen at www.earlybirddiabetestrust.org