The government has let down future generations with a watered-down strategy that will fail to adequately tackle the rising swell of obesity in the country, campaigners say.

A third of children are already overweight or obese by the time they leave primary school, and, based on current trends, half of all children will be obese or overweight by 2020. The NHS in England already spends more than £5 billion a year on overweight and obesity-related ill-health.

It had been hoped that the long-awaited childhood obesity strategy would follow recommendations from Public Health England, particularly curbs on junk food advertising and multi-buy promotions to help address the crisis, neither of which feature in the strategy.

"Even after delaying publication for a whole year, the government has fallen far short of what is needed and is failing to take the necessary measures to tackle childhood obesity," said the Obesity Health Alliance, a coalition of 33 leading national charities, Medical Royal Colleges, and campaign groups.

Under the plan, the government is to introduce a soft drinks industry levy on producers and importers across the UK, designed to encourage a reduction in the amount of sugar contained in these products. In England, the revenue from the levy will be invested in programmes to reduce obesity and encourage physical activity and balanced diets for school age children.

It will also launch a "broad, structured sugar reduction programme" to remove sugar from the products children eat most, with all sectors of the food and drinks industry challenged to - voluntarily - reduce overall sugar across a range of products that contribute to children's sugar intakes by at least 20 percent by 2020, including a 5 percent reduction in year one.

However, the measures on their own "will not tackle the obesity crisis and are not sufficient to reduce the rising toll of ill-health, premature deaths and unsustainable costs to the NHS," the Alliance warns.

The group welcomed the government's soft drinks industry levy consultation "as a bold and positive step forward," but stressed that the plan outlined "lacks ambition and will fail to tackle childhood obesity as promised in the Government's manifesto".

"This strategy was Britain's opportunity to lead the way and to implement real, meaningful environmental change, to start removing the crippling financial burden from our NHS and reversing the tide of diet-related disease," said celebrity chef and healthy eating campaigner Jamie Oliver. "With this disappointing, and frankly, underwhelming strategy the health of our future generations remains at stake".

Dr Helen Stokes-Lampard, spokesperson for the Royal College of GPs, also said "it's disappointing that some of the more radical initiatives expected, such as a ban on pre-watershed junk food advertising, that could potentially help to curb rising levels of childhood obesity, don't feature in today's report," but she also argued that the measures that do "are positive steps forward".

"After the farce of the responsibility deal where Andrew Lansley made the food industry responsible for policing themselves, it is sad to see that this is just another imitation of the same responsibility deal take two," said Prof Graham MacGregor, chairman of Action on Sugar and Consensus Action on Salt and Health, as reported by the Guardian.

"It is an insulting response to the UK crisis in type-2 diabetes, both in children and adults. This will bankrupt the NHS unless something radical is done."