Campaigners have called for a 20% sales tax on confectionery as well as a complete ban on price promotions in response to retailers making larger 'sharing packs' better value than smaller sizes.
Action on Sugar (AoS) said some chocolate confectionery sharing bags contained 29 teaspoons of sugar - four times an adult's maximum daily sugar intake.
However, more than one in five adults (22%) eat the sharing bags alone, in one go, with the figure rising to 35% among 16 to 24-year-olds, a survey for AoS found.
Confectionery consumption is the second highest contributor to sugar intake in children after soft drinks.
AoS said the removal of price promotions on high sugar products could cut almost two teaspoons or 7g of sugar on average from every individual's diet per day.
At the time of its survey in December and this month, AoS found Tesco, Asda, Sainsbury's, Morrisons, Co-op and Waitrose were all offering price promotions on some sharing bags, selling them for as little as £1 and making them comparatively better value for money than smaller pack sizes.
The campaign group is calling on the Government to "immediately revise and strengthen" its childhood obesity plan.
Kawther Hashem, nutritionist and researcher at AoS based at Queen Mary University of London, said: "Companies and supermarkets are constantly finding ways to push more sugary products, which are contributing to the high rates of obesity, Type 2 diabetes and tooth decay in the UK.
"These types of price promotions encourage us to eat far too much sugar and calories and should be banned. It is time retailers are pressed to act responsibly and no longer profit at the expense of our health."
AoS chairman and professor of cardiovascular medicine at Queen Mary University of London, Graham MacGregor, said: "It is shocking that food companies are being allowed to exploit consumers by manipulating them into purchasing larger size bags of chocolate confectionery on the cheap.
"Theresa May is letting companies get away with this despite pledging to help the socially deprived when she first became the Prime Minster.
"Companies must be held accountable and reminded to reconsider their ethical and corporate responsibility."
A Department of Health and Social Care spokesman said: "Current advertising restrictions in the UK on junk food are among the toughest in the world, including a ban on advertising junk food in children's media.
"Alongside this we are delivering the most ambitious childhood obesity plan in the world--taxing sugary drinks, funding further research and cutting sugar and calories in food before it hits shelves and plates.
"But it's very early days tackling a problem decades in the making, and we have not ruled out further action if the right results are not seen."