The UK's obesity crisis is being fuelled by businesses pushing unhealthy food and larger portions on shoppers, according to health experts.
The Royal Society for Public Health warned consumers were being tricked by a marketing ploy known as upselling.
The tactic involves shops, cafes and restaurants encouraging customers to upgrade to larger meals and drinks or adding high-calorie toppings and sides.
A poll suggested eight in 10 people experienced it every week.
The most common upsells to be taken included larger coffees, bigger meals, sweets and chocolates and extra sides such as onion rings and chips.
Royal Society for Public Health chief executive Shirley Cramer said the industry was pressuring the public into buying extra calories, which then added up "without us noticing".
She said businesses needed to stop training staff to upsell high-calorie food and instead focus on healthy alternatives.
The findings were drawn from a poll of more than 2,000 UK adults by the RSPH and Slimming World.
Those who had experienced upsells had been targeted more than twice a week on average, with younger people the most susceptible.
The most common place for it to happen was restaurants, followed by fast-food outlets, supermarkets, coffee shops and pubs and bars.
The research showed many of the upsells were unhealthy options, with the average person who fell victim to the technique consuming an average of 17,000 extra calories a year, enough to gain an extra 5lbs (2.3kg) over 12 months.
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'I kept falling for upsells'
Liam Smith, 25, from West Yorkshire, is just one of the many people who have been persuaded by the marketing ploy.
But since recognising he was eating too much he has lost 6st (38kg) and now refuses upsells.
"Being able to 'go large' on a meal for 30p extra was always a no-brainer for me, as was a few pence more for a large cup of hot chocolate or paying £1 more to turn a single burger into a double.
"Afterwards, I'd wish I hadn't done it though - I can only describe it as a major feeling of guilt."
The top 10 places for upselling
- Fast-food outlets
- Coffee shops
- Pubs and bars
- Planes and airports
- Petrol stations
- Railway stations
The practice occurs at the point-of-sale and is not at the customer's request.
Examples include a coffee shop barista asking if you would like a large rather than a regular-sized coffee or if you want whipped cream added.
Another popular one is a fast-food worker asking if you would like to make a meal large for only a minimal cost.
One worker told researchers: "I've been trained so that if a customer asks for a product, I always ask if they'd like to make that a meal."
Some bar workers are also trained to nudge people towards buying a double rather than a single measure.
But Brigid Simmonds, chief executive of the British Beer and Pub Association, said there was "absolutely no evidence" of upselling in pubs.
"Telling people what to do is not what you do," she told BBC Radio 4's Today programme. "You go to a pub and there's a certain amount of free choice."
She adds: "There are actually fewer calories in half a pint of beer than there are in a glass of orange juice."
And Andrew Opie, of the British Retail Consortium, also disputed that supermarkets upsold.
He said: "They promote and market products in store, but ensure there is a balance of products and it is offered as choice rather than upselling."