Only one in three ten-year-olds in the West Midlands agreed to be An weighed by experts trying to build a new 'obesity database.'
Children aged four to five-years-old in reception classes, and Year 6 pupils aged ten or 11, were due to be measured for the Government's new National Childhood Obesity Database (NCOD).
But children were allowed to opt out and there were higher opt-out rates among heavier children, leading to results that campaigners say are useless.
The report, Analysis of the National Childhood Obesity Database 2005/06, compiled by the Association of Public Health Observatories for the Department of Health also reveals poor response rates from primary care trusts (PCTs), who were tasked to collect the information.
In the West Midlands 62,600 pupils were weighed part of the exercise, with nearly one in three (27.8 per cent) classified as obese according to their Body Mass Index readings.
A total of 38,900 four and five-year-olds (67.5 per cent) and 23,700 (36.4 per cent) were weighed and measured in the region.
Of the reception pupils 3,967 (10.2 per cent) were obese, and that rose to 4,171 (17.6 per cent) in Year 6 children.
Janet Baker, deputy director of public health for the West Midlands, admitted it was now clear that measurements "should be a requirement, not an option".
"A lot of parents, particularly those with children in Year 6, refused to allow their sons or daughters to be weighed, whether they were overweight or not," she said.
"Next year we hope to measure more than 90 per cent of all children, to get a truer picture of childhood obesity in our region.
"We've got to make healthy options and exercise appear to be the norm, particularly as these children get older, rather than junk food and sitting in front of the television or playing computer games instead of sports."
Recent Government figures revealed the West Midlands had the highest percentage of overweight women in the country, with 29 per cent classified as obese, compared to 23 per cent of men in the region.
Ms Baker added: "If this current trend continues we could see obesity levels rise above 30 per cent, which is why we're working so hard to break this cycle."
Tam Fry, board member of the National Obesity Forum, claimed the exercise had been a "waste of time."
In January, the Government issued guidance to PCTs saying they should measure "all primary school children in the reception year, ages four to five years" and "all primary school children in Year 6, ages 10 to 11 years".
Today’s report said 538,400 children in both years were measured - about 48 per cent of those eligible. Of those children that were measured, 12.3 per cent of girls and 13.4 per cent of boys in the reception year were found to be overweight and 9.2 per cent of boys and 10.7 per cent of girls were obese.
In Year 6, 13.8 per cent of boys and girls were overweight and 15.4 per cent of girls and 18.9 per cent of boys were obese.
Overall, 80 per cent of PCTs returned some data on schools in their area but the report stated response rates "varied widely across England."
A Department of Health spokeswoman added: "Parents were formally advised about the measurement exercise and could opt out."