One in three people buys a larger coffee, more fries or added cream each week as a result of "upselling", which experts say is fuelling the obesity epidemic.

Most people - 78 per cent of respondents in a survey - say they are asked in restaurants, fast-food outlets and stores if they want to opt for more food or bigger portions.

Those who succumb to upselling get 55 per cent more calories by paying just a fraction more, says the report.

Those calories add up to a weight gain of 2.2kg every year.

People aged 18 to 24 are the most likely to experience upselling, consuming an extra 750 calories a week that could potentially lead them to put on 5kg in a year.

The report comes from the Royal Society for Public Health (RSPH), together with Slimming World, who say upselling is a technique to get customers to spend more.

Public health experts have called for action against buy-one-get-one- free promotions in supermarkets.

But upselling, the act of persuading a customer to buy something additional or more expensive, has gone under the radar, says the report. "Studies have shown that when people are presented with larger portion sizes, they consume more and increase their calorie intake."

Restaurant staff describe in the report how they are trained and given incentives to upsell.

One cafe worker says that if a customer asks for a latte, "I will reply with 'large'? ...insinuating that a large is, in fact, what a typical customer orders often nudges the customer into ordering the bigger and slightly more expensive drink".

A fast-food worker says that staff are trained to always ask if the customer would like a meal. "We get reminded of how many large meals we are expected to sell."

A pub worker alerts customers to offers, such as "Would you like a portion of chips? They're half- price", and a table of two people will often end up with an extra plate.

"The major incentive for upselling is a team competition, with a prize for the winning team," he says.

The report urges the government to cut business rates for outlets that undertake not to train staff to upsell unhealthy food and drink, or link staff payments to upselling.

RSPH chief executive Shirley Cramer said: "Obesity is the public health challenge of our generation and, if not addressed urgently, could tip over the point of no return."

Ms Caroline Cerny of the Obesity Health Alliance, a coalition of over 40 organisations with an interest in the issue, said: "It's all too easy to eat more than we need when we are encouraged to buy larger sizes, add unhealthy extras or take advantage of special offers.

"Marketing techniques persuade us to eat and drink more of the wrong types of food and this is driving the obesity epidemic."

THE GUARDIAN