Food manufacturers have been urged to tackle obesity by reducing the size of snacks and soft drinks.

Health minister Dawn Primarolo said the fact that 30 per cent of primary-school children were overweight meant that firms should start undersizing products.

Speaking at a conference organised by the Department of Health and the Food Standards Agency, Ms Primarolo said she wanted to cut levels of saturated fat by 20 per cent.

"Most companies do offer a range of sizes, which is great," she said. "But are snack-sized options available where people make impulse buys? And are they sold individually, not just in multi-packs?"

She urged restaurants to list the calories in dishes, saying that this practice had worked in New York.

The British Hospitality Association said that introducing calorie counts on menus would be "impossible for most restaurateurs".

One in four adults is currently obese. By 2050, the proportion is expected to rise to one in two.