The extra calories could mean a potential weight gain of five pounds, the report said.Young people aged 18 to 24 are the most likely to experience up-selling, with the study finding that they consume an extra 750 calories a week as a result, which could mean an annual gain of up to 11 pounds.The charity urged businesses to pledge to only up-sell healthy food and drink, and said shops should stop paying staff commission for hitting “up-selling” targets.The research found that those who were persuaded to supersize on average spent 17 per cent more for 55 per cent more calories, with special offers reserved for the least healthy fare. 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.”Almost everyone can relate to the feeling of being pressured into buying extra calories through up-selling.”The charity and diet company are urging consumers to use the hashtag #JustThisThanks to fight back against the trend, which the head of the NHS has previously described as “a form of health pollution”. On average, consumers will face 106 attempts at upselling a year, the study found Credit:Dominic Lipinski/PA Supersize tactics by retailers mean the average person is consuming an extra 17,000 calories a year – which could mean five pounds weight gain – health experts have warned.The Royal Society of Public Health said consumers are facing more than 100 attempts each year by shops and fast-food chains to “up-sell” unhealthy foods and drinks each year.The charity said soaring obesity levels were being fuelled by pushy sales assistants, trained to ask customers if they wanted to “go large,” upgrade to a meal deal, or add cut-price chocolate to their purchase.It urged retailers to stop linking staff pay to the success of efforts to pressure customers into buying more junk food.The report carried out with Slimming World, found that over the course of a week, “verbal pushes” meant 34 per cent of customers ended up buying a larger coffee than requested, with 33 per cent upgrading to meal deals, and 36 per cent adding chocolate to their shop.The report, from a survey of more than 2,000 UK adults, found that consumers face an average of 106 verbal pushes annually, which led to an extra 330 calories a week, or 17,000 calories a year. Healthy food is rarely pushed on customers Credit:Nick Ansell /PA Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.