Hundreds of children may have been wrongly told they are overweight because of a "software error", the NHS Information Centre said today.
The fault is thought to have affected a maximum of 2,500 children whose height and weight was recorded last month as part of the National Child Measurement Programme.
But the NHS Information Centre said the likelihood is far fewer letters will have already been sent out as it apologised to all parents who have been sent the wrong information about their child.
Chief executive Tim Straughan said: "Information about their child's weight is a sensitive and important matter for parents and we would like to apologise for any unnecessary concern this error may have caused.
"We alerted NHS staff responsible for carrying out the measurement programme locally as soon as the problem came to light and are working with them to ensure that families affected by these letters are contacted personally.
"We are fixing the software fault as quickly as possible."
The inaccurate letters were sent to parents of children in reception and Year 6.
While they include the correct information about children's height, weight and Body Mass Index (BMI), the letters assign children to the wrong BMI category.
As a result some children of a healthy weight have been classed as overweight and some who are overweight have been described as very overweight.
Local measurement programme administrators at primary care trusts have been asked to tell the parents affected their child's correct results as soon as possible.
Health Secretary Andrew Lansley said: "I am sorry that these mistakes occurred.
"The Information Centre have rightly expressed their apology for this.
"I support their action now in contacting all the families affected, and their efforts, alongside PCTs, to work together to put it right."
Tam Fry, spokesman for both the Child Growth Foundation and the National Obesity Forum, said there was also concern about the wording of the letters which warns that children who are overweight could develop high blood pressure or the early signs of type two diabetes and have a greater risk of heart disease and some types of cancer in later life.
"We have argued with the Department of Health for years about these letters," he said.
"Not only are they wrongly worded but some 10% should be vetted first by the school to ensure that this kind of catastrophe does not happen.
"It may be a little extra work for the school nurse to do but, as health professionals and mothers, they will know only too well the effect that such news might have on caring families."
The National Child Measurement Programme measures around one million children a year.