Marketing junk food to children has to become socially unacceptable, a leading obesity expert will say today, warning that the food industry has done too little voluntarily to help avert what a major report this week will show is a "far worse scenario than even our gloomiest predictions".

Professor Philip James, chairman of the International Obesity Taskforce thinktank, will say in a keynote speech that when it comes to public health the response of the food industry has so far been a case of "too little, too late".

Today's conference of the National Obesity Forum comes on the eve of publication of a government-commissioned investigation into the scale of the obesity problem and what science can do to help tackle it.

"The expected outcome of the Foresight Report on Obesity suggests we are facing a far worse scenario than even our gloomiest predictions," Professor James will say, calling for a cabinet overlord to lead the cross-departmental fight, involving changes in education, media, culture, transport, leisure, and the food chain as well as in health service provisions.

He will also call for negotiation with the food industry to establish targets for improvements. "Our diet-related health should no longer be a casualty in a battleground where every advance is resisted to defend short-term market share and profit. The food business will do best with clearly agreed goals on changes to our foods," he will say.

Banning the advertising of junk food during children's television programmes is not enough, he will say. All kinds of marketing must be addressed. "We must go much further in protecting children ... we need to make it socially unacceptable to peddle to children and that means big supermarkets and small retailers really changing their approach."

The Foresight report on Wednesday is expected to say that obesity will cost the country £45bn a year by 2050 if the loss of productivity from people who suffer obesity-related healthcare problems is added to the cost of treating them. By that time 60% of men, 50% of women, 20% of primary school girls and half the boys will be obese if more is not done, it will suggest.

The government sees this week's anti-obesity push, with the Foresight report at its centre, as an example of thinking long-term, working out the problems of 25-30 years ahead and trying to solve them.

But many in public health feel that there has been more target-setting and talk from the government than useful action.

Another speaker at today's conference will say that weighing and measuring children in schools, a key initiative of the government's anti-obesity strategy, is a waste of time unless school nurses are given the resources to help overweight children.

Ros Godson, a leading member of the Community Practitioners and Health Visitors Association, which represents the country's 2,500 school nurses, will say that the programme needs to be linked to help inside and outside the classroom with diet and exercise if it is to make a difference. "Just weighing and measuring children is a precious waste of time; it provides no support to those that need help," she will say.

"Ideally we need to tie it in with the work done in class, with the Healthy Schools Programme, to find a way for the health and education disciplines to work together and secure a robust prevention, treatment and referral system."

A third of children are now leaving primary school overweight, she said. "School nurses already play a vital role in the fight against childhood obesity and are ideally placed to support the national effort to reduce it. There is a need for health education in the home and at school but as the situation stands we have neither the team, time, resources or remit to maximise this opportunity."

The government has set an ambitious target to halt and begin to reverse the tide of childhood obesity by 2010, but all the evidence is of an apparently inexorable increase. Some expect the target will be pushed back to 2015 or even 2020.

Critics say planned initiatives, such as curbs on advertising junk food to children, have either not yet begun or have yet to make any impact. Others are focused on very small groups of obese children and will not turn around the trend towards fat across the country.

The Liberal Democrats released figures yesterday showing that hospitals had doubled their spending on equipment for obese patients - up to £60,000 this year compared with £30,000 three years ago. The money would be better spent tackling the problem, said health spokesman Norman Lamb.

"Hospitals are doing what they must to take care of their patients. It is the government that must take responsibility for failing to do enough to halt the rise of this public health crisis. We now need urgent action to encourage healthier eating from the government and food industry."

All in the mind

Scientists have identified two crucial brain regions that control how hungry we feel, which could one day lead to drug treatments for obesity. Traditionally, scientists have attributed appetite control to a region of the brain stem called the hypothalamus. Rachel Batterham at University College London and her team tested eight normal-sized men in a brain scanner on two separate occasions and found that a brain region involved in pleasure and reward, the orbital frontal cortex, is also involved.