The government's pledge to halt the soaring rate of childhood obesity within three years in fact cannot be achieved until 2050, warns a damning report commissioned by ministers to help them tackle the problem.

The number of six- to 10-year-olds who become obese will keep rising relentlessly until the late 2040s, with as many as half of all primary school-age boys and one in five girls dangerously overweight by 2050, according to the document.

Health campaigners last night said the dramatic findings showed that Britain is facing an unprecedented public health crisis. 'They are frightening preliminary findings and the implications for both the NHS's budget and, more importantly, for children's health are really disturbing,' said Norman Lamb, the Liberal Democrats' shadow health secretary. 'I'm not entirely surprised by the direction of the trend but I'm shocked by the scale of it, which is horrifying.'

The confidential document was commissioned by the former Department of Trade and Industry. The paper predicts that both adult and childhood obesity in England will continue rising unless there are dramatic changes in official policy and people's lifestyles. It foresees big increases in heart problems, strokes and diabetes if current upward trends in obesity are not reversed. Few experts expect that to happen.

The 74-page draft document, dated 22 June 2007, also estimates the NHS will have to spend billions more pounds every year on treating patients with obesity-related illnesses. 'A target on childhood obesity was introduced to halt the year-on-year rise in obesity among children aged under 11 by 2010 in the context of a broader strategy to tackle obesity in the population as a whole. As yet there is almost no evidence that the rise in obesity levels is changing.'

And the report goes on to say: 'There is evidence that among children aged six to 10 years, males will be more obese than females, with an estimate of 50 per cent being obese by 2050, compared with 20 per cent of female children. Among children aged 11 to 15, the prediction is different: 23 per cent for males and 37 per cent for females.'

At present around one in 10 under-10s, and one in four of 11- to 15-year-olds, are classed as obese: that is, having a Body Mass Index of 30 or above.

One graph in the report shows that while the number of boys aged six to 10 who are merely overweight, with a BMI of 25 to 30, is expected to stay constant until 2050, the number who are obese will soar, meaning the number with a healthy BMI rating will decrease sharply. An identical graph for girls between six and 10 shows the number who are overweight or obese also rising steadily until 2050.

The report on potential future trends in obesity was put together by a team of researchers headed by Professor Klim McPherson, a professor of public health epidemiology at Oxford University. Using data from the official Health Survey for England and the widely respected International Obesity Task Force, McPherson's report predicts that the population as a whole will continue getting fatter, with efforts to curb the epidemic of youthful obesity bringing at best limited success.

The Department of Health last night refused to respond to the McPherson report's findings because it never discusses leaked documents. But officials describe the 2010 public service agreement target, set in 2004, as 'very challenging'.

National Obesity Forum spokesman Tam Fry said: 'Unless we take proper steps now to tackle it, we are facing disaster in the near future, with today's generation of children dying younger than their parents.'