Hundreds of children may have been wrongly told they are overweight because of a "software error", the NHS Information Centre has said.
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.
The National Child Measurement Programme measures around one million children a year.