Cutting the amount of sugar in sweetened drinks by 40% could prevent 300,000 cases of diabetes and one million cases of obesity, research suggests.
The study, published in The Lancet Diabetes and Endocrinology journal, states that this reduction in sugar-sweetened beverages (SSBs) - including fruit juices - over five years could lead to 500,000 fewer cases of people being overweight, one million fewer cases of obesity, and 300,000 fewer instances of Type 2 diabetes, over two decades.
Some fizzy drinks contain around 10 teaspoons of added sugar.
In 2014, the World Health Organisation (WHO) said people should cut their sugar intake in half if they wanted to reap health benefits.
It said sugars should make up less than 5% of total energy intake per day for adults and children.
The Lancet study was led by Professor Graham MacGregor, chairman of the Action on Sugar group, who said its action plan needed to be backed up by a Government-funded but independent nutrition agency "which can set mandatory targets with robust enforcement".
He added: "In support of this, the British Retail Consortium is now calling for regulated sugar, fat and salt reduction targets.
"The UK food and drink industry could lead the world in preventing obesity and type 2 diabetes."
In the study, researchers used data to calculate the level of SSB consumption and its contribution to energy intake in the UK population.
They then estimated how the reduction would affect body weight per person, and overall results for the adult population.
These calculations showed that a 40% reduction in sugars added to SSBs over five years - provided they are not replaced by artificial sweeteners - would lead to an average reduction in energy intake of 38.4 calories per day by the end of the fifth year, in turn leading to an average 1.2kg reduction in adult body weight.
This in turn would prevent more than a million cases of disease.
The predicted impact was found to be greater in adolescents, young adults and individuals from low-income families who consume more SSBs, the research said.
But researchers said that the gradual sugar content reduction would not affect sales or product prices.
Professor Stephen O'Rahilly, from the University of Cambridge, said: "This is a purely theoretical study and there are many assumptions made which reduce confidence in the statements regarding the precise extent of the health benefit."
He added that, while the evidence surrounding the health dangers of sugar are "hard to argue with", the same does not necessarily apply for replacement sweeteners.
He said: "The best quality evidence currently available indicates that artificial sweeteners do not significantly contribute to human obesity or diabetes.
"It might also be easier to incentivise companies to change the emphasis of their manufacturing and marketing toward versions of their current products that contain non-caloric sweeteners rather than to unsweetened drinks such as bottled water."
Tam Fry, from the National Obesity Forum, said: "It is paramount that everyone in the UK strives to reduce their sugar consumption by half. That's easier said than done but the 40% reformulation proposed by Action on Sugar would make a significant start.
"As the paper states, people will need time to adjust their taste buds to accept a lower sugar hit and the forum has reservations that such a dramatic cut could be achieved by 2022."
The group also stressed concern that SSB producers would be reluctant to get on board with the strategy.
Gavin Partington, director-general of the British Soft Drinks Association, suggested sugar is not the only thing to blame for rising levels of obesity and that measures such as taxing soft drinks would have little effect.
He said: "Sugar consumption in the UK has been declining for many years, particularly from soft drinks, as Government statistics testify, while levels of obesity increased.
"There is also no evidence that a tax on soft drinks would have an impact on obesity. The soft drinks tax in Mexico has reduced average calorie intake by just 6 calories a day and sales in France are back to pre-tax levels.
"By contrast, the soft drinks industry is taking practical steps to help consumers, through reformulation, smaller portion sizes and increased promotion of low- and no-calorie options - reducing sugar intake by 7.5% in recent years, and with plans to reduce calories by a further 20% by 2020."
Public Health England (PHE) has backed stronger measures to control sugar content in drinks.
PHE chief nutritionist Alison Tedstone said: "Sugary drinks are the biggest source of sugar in young people's diets.
"We believe a programme to reduce the sugar from the sweetest drinks - alongside other measures like controls on advertising and marketing - would lead to a significant drop in the amount of calories consumed. As a result, we'll stand a better chance of tackling child obesity and tooth decay."
Diabetes UK said: "Reducing sugar content in soft drinks alone is not enough to achieve the reduction in sugar intake we need to see across the population. We need to see the Government act on recommendations made by Public Health England, including restricting marketing of unhealthy foods to children.
"People also need to be supported to undertake regular physical activity and supported to choose healthier foods, including through a clear and consistent food labelling system. Until these recommendations are implemented, we will continue to see cases of Type 2 diabetes soar at an alarming rate, costing not only human lives but also crippling the already overstretched NHS."