Calls are growing for greater regulation of junk food, with experts saying the government is doing too little to tackle obesity and is playing into the hands of the food and drink industry by relying on it to curb unhealthy ingredients and refrain from promoting its products to children.

Dr Tim Lobstein, director of policy and programmes at the International Association for the Study of Obesity (IASO), said voluntary agreements, such as the government's "responsibility deal" with food and drink companies, offered more benefit to the industry than the public. "It is a very valuable opportunity for the industry to play for time," Lobstein said. "It leads mainly to tinkering at the edges of the food supply. It doesn't really change the overall environment."

He said the industry got involved so it could pre-empt stronger regulation, such as a soft drinks tax or a ban on advertising junk food before 9pm, measures proposed in a report by the Academy of Medical Royal Colleges on Monday. Companies liked to be seen as part of the solution, Lobstein said, but "it would be unheard of for the tobacco industry to be sitting round the table".

A spokesman for the Department of Health said the responsibility deal, in which it now had more than 500 partners, had helped reduce fat, sugar and salt in foods. "Our successes so far clearly demonstrate that the voluntary approach can work … but we're not complacent and we are clear this is something for all food businesses, not just some, and if we don't get continued progress we will have to consider alternative approaches."

The National Obesity Forum said there has been too little progress because of a lack of government interest. David Haslam, the forum's chair and a Hertfordshire GP, said: "I don't think enough is being done by the responsibility deal. There is a massive degree of urgency about this. The problem is the political leadership up to now have been frankly useless. The government just isn't interested."

Haslam said obese children should be offered stomach surgery to help them reduce their weight to safer levels. "I'm perfectly happy to call it a radical approach because it is a radical approach," he told Channel 4 News. "But it's needed in desperate situations. If a child or adolescent is sitting in front of us and we know the child is going to lose 20, maybe 30 years, of life because of their obesity and the conditions it will lead to, then an operation to increase life expectancy by decades – I think that is well worth doing."

The IASO recently looked at junk food advertising in Europe and found that in spite of promises from the food industry, children's exposure to ads for fatty and sugary products had fallen by barely a quarter in six years.

Professor Terence Stephenson, chair of the Academy of Medical Royal Colleges, said voluntary agreements did not work with the tobacco industry, and bans and regulation were necessary. "Asking the food and drink industry to voluntarily promote healthy living when much of the food they are selling is unhealthy is simply unrealistic," he said. "What's the incentive for fast food manufacturers to promote healthy eating when they're making huge profits from high-calorie foods and drinks? It's like asking petrol companies to encourage people to cycle or walk rather than use their cars.

"We heard evidence from a range of individuals and organisations who felt this approach simply doesn't work.

"You might get some small 'victories' and gradual change, such as the well-publicised reduction in the amount of salt in foods. Whilst these should be applauded, if we are to make a significant impact – which is what is needed given we are facing a public health crisis in the form of obesity – there need to be more aggressive measures."

The consumer group Which? sits on the steering committee of the food responsibility deal, but said it was not moving fast enough. It is calling for companies that drag their feet to be named and shamed. Sue Davies, the group's chief policy adviser on food, said: "With a quarter of the population classed as obese, it's vitally important that people know what's in their food so they can make an informed choice. The government needs to set more ambitious pledges to include clearer targets for supermarkets and manufacturers in areas such as reducing saturated fat, sugar and salt and ensuring responsible promotions."

On Tuesday, the Soft Drinks Association hit back at the call for a fizzy drinks tax, saying they accounted for just 2% of calories in an average diet and that sales had fallen in the last decade.

The Food and Drink Federation said it believed that collaboration between many different organisations was needed to tackle the health burden of obesity and diet-related diseases. In the responsibility deal, said Barbara Gallani, director of food safety, science and health, "UK food and drink manufacturers are working in partnership with government, health organisations, NGOs and other stakeholders.

"Voluntary frameworks that have been demonstrated to work include the elimination of partially hydrogenated vegetable oils as ingredients in foods and a 10% reduction in salt levels in FDF members' products over the past five years," she said.