Consumers are "inundated" with chocolate offers, said Mr Cameron

In a health speech, Mr Cameron pointed to shops, including at train stations, offering cut-price chocolate bars.

He asked: "As Britain faces an obesity crisis, why does WH Smith's promote half-price Chocolate Oranges at its checkouts instead of real oranges?"

WH Smith denies acting irresponsibly and says it offers customers choice.

A spokeswoman said dried fruit and other products, as well as chocolate, were on sale in its outlets.

And promotions varied - with water among the healthy options promoted in the past, she said.

"They are marketed at adults," she said. "It's about choice - you don't have to purchase the products."

Double-edged sword

He told the King's Fund in London that business had a shared responsibility to help improve public health.

"Modern marketing techniques can be used to great effect to tackle the root causes of preventable ill-health... just as irresponsible marketing techniques can have the opposite effect," he said.

"Try and buy a newspaper at the train station and, as you queue to pay, you're surrounded, you're inundated by cut price offers for giant chocolate bars."

Mr Cameron said many shops could do more to promote healthy diets and lifestyles - and that was good business because it reduced sickness costs.