Supermarket multi-buys and special offers are driving the obesity crisis, experts have warned.
The discounts and deals boost the amount of junk food consumers buy while cutting their purchases of fruit and vegetables, they said.
Nearly a third of the food and drink in trolleys is bought under various promotions such as buy-one-get-one-free, according to a study of the shopping habits of more than 16,000 people.
It found that consumers who buy the most food this way are 54 per cent more likely to be obese than those who shun promotions.
Shoppers who opt for lots of special offers also buy a fifth more unhealthy food and drink, regarded as being high in fat, sugar or salt.
The research by Cancer Research UK found that a promotion-loving family with two adults and two children buys an extra 11 unhealthy items per month.
Bargain-lovers also buy more sugary items and fewer with fibre than low promotional shoppers.
Consumers who favour deals choose nearly a third fewer fruit products and a quarter fewer vegetables, items which are usually not on promotion.
The report found that nearly half of chocolate, crisps, popcorn and savoury snacks bought in supermarkets have been discounted or promoted in some way.