Parents will then be told that if they need more advice, they should consult their GP, school nurse or practice nurse. Children will not be given the results, to prevent them being stigmatised by other children.

Dr Cavendish admitted that parents could, if they wanted, throw the letters away. PCTs will take no further action to ensure that the advice is acted upon. So far, only 40 per cent of PCTs have undertaken to send the letters. Another 40 per cent are considering whether to do so, and are expected to make their decisions in the light of today’s advice.

The data will be shared only with parents and not with school staff, the child, other children or, directly, with the child’s GP.

Ivan Lewis, the Health Minister, said: “It’s clear from research that we’ve done that parents want to know their child’s results and whether there is concern about their health.

“But they want clear information which is helpful and nonstigmatising.

“Today we’ve published guidance which will help PCTs deliver this programme and help to make sure parents get the information they need about their child’s results in ways that they have said will be most helpful.

“Research shows that most parents of overweight or obese children think that their child is a healthy weight.

“This important move isn’t about pointing the finger and telling parents that their children are overweight. Instead it’s about equipping parents with the information they need to help their children live healthier lives.”

Mr Fry said that the Department of Health’s decision not to tell parents the exact BMI of their child was “rubbish”.

“Although the department recognises that there is no better way of calculating healthy/unhealthy weight other than using BMI, and mistakenly states that BMI growth charts can be found in the child’s personal child health records, it states that ‘some parents might find BMI charts confusing’.

“The Department of Health’s solution? Not to give anyone a BMI figure for their children in either reception year or Year 6. This flies in the face of the department’s policy recognising that parents are the principal care givers, yet denies them knowledge.”

The Conservatives gave warning that a lack of school nurses threatened the success of the programme.

Andrew Lansley, the Shadow Health Secretary, said: “If we are to avoid stigmatising children after being weighed then there needs to be sympathetic follow-up care but Labour have failed to address the chronic shortage of school nurses.

Sandra Gidley, the Liberal Democrat health spokeswoman, said: “The Government is clearly pussy-footing around this issue. Unless these letters are accompanied by practical help, then they will be a waste of time and resources.”