More than 40% of men and women in the UK could be obese within a single generation, the influential International Obesity Taskforce will warn at a conference today.

The warning follows publication of a dossier yesterday by three research organisations designed to highlight the urgent need to stem an obesity "epidemic" in children. Susan Jebb, the head of nutrition and health research at the Medical Research Council's human nutrition research unit, called for the government to act and set real targets for bringing obesity levels down.

"Many organisations feel that government has to set the framework for tackling obesity. We probably do need to set targets and certainly hold groups accountable for meeting these targets."

She said that it should be easier to tackle the problem in childhood than among adults. "Most children are not fat. It is not too late if we act now. In adults we are just 10 years behind the USA and that is nothing at all. Ten years ago we all thought Americans were huge and that is the situation here now. We seem to be just following in the footsteps of the United States.

"I think with adults it is very easy to feel quite pessimistic. One in five adults is clinically obese, and two in five are overweight. That makes it a huge mountain to climb. Perhaps the best we can do is stop it getting worse.

"But children are still growing. Even those who are overweight don't have to lose weight because they are still growing - we need to contain their weight gain, so they will grow into it."

At the IOTF conference today its president, Philip James, will say there is overwhelming evidence of serious health risks to children, and decisive action is necessary.

"The evidence is so compelling that we must now act rapidly and decisively to deal with this epidemic. The first step must be to start protecting the health and wellbeing of our young children, who are being damaged because we are not yet willing to provide them with a safe environment where they can learn the value of good food and play.

"Instead, their wellbeing is systematically undermined by the intense marketing and sales of foods and drinks high in fat, sugar and salt. But we also need to develop effective strategies that can be made to work throughout society to help everyone address the problem of obesity," Professor James will say.

He will warn that the close association of confectionery and soft drinks companies with the promotion of physical activity and sport is sending the wrong messages. Young people are being encouraged to over-consume products that contribute to their increasing weight, while believing they are emulating sports idols.

Prof James will hit out at the Food Standards Agency, which is consulting about a ban on advertising food and drink to children, but has left open the possibility that it could be voluntary.

"We need even stronger action to deal with marketing to children and not some fudged option such as those just suggested as options by the Food Standards Agency.

"Politicians need to understand that regulation is required and that it is a popular move. It is what the majority of parents in every country we work with place as one of their top priorities. The issue is then whether governments, local authorities and schools are working for the people or for commercial interests."

· The public health minister, Melanie Johnson, gave the food industry a pointed warning yesterday that she was prepared to get tough on the issue of excess salt in food. Retailers, manufacturers and caterers may be forced to label some of their products "high in salt" if they do not bring down the levels in processed food by February.