Food packets should warn how much exercise is needed to burn off the calories consumed, an expert has said.

Packaging should not only provide nutritional information but should "also help people to change behaviour," said Shirley Cramer, chief executive of the Royal Society of Public Health.

There is "little evidence" that traffic light labelling actually changes behaviour, she said.

In a personal view, published in The BMJ, Ms Cramer said food and drinks should be labelled with "activity equivalent" calorie information.

This should include how much time people could spend doing various different exercises that are "equivalent in the calories expended to those in the product", she said.

Such information would help people to choose healthier products, eat smaller portions or do more exercise.

"The aim is to prompt people to be more mindful of the energy they consume and how these calories relate to activities in their everyday lives, to encourage them to be more physically active," she wrote.

She points to research which found that 44% of people find current front-of-pack information confusing.