Scotland's planned flat-rate deposit return scheme for drinks cans and bottles could "severely hamper" efforts to tackle childhood obesity, public health experts have warned.
Setting a fixed rate of 20p for all containers, regardless of their size, could encourage more people to buy larger bottles of sugary drinks, they claimed.
The group, which includes senior figures from Glasgow and Strathclyde universities, as well as the London School of Economics, argued instead that there should be a graduated charge under the deposit return scheme, with this rising in line with the size of the drink container.
Scotland is the first part of the UK to have announced plans to bring in a deposit return scheme in a bid to boost recycling.
Under proposals to be brought in before the end of the current parliament in March 2021, a 20p charge will be levied on the vast majority of drinks containers, including PET plastic bottles - used for fizzy drinks and water bottles - glass bottles and steel/aluminium drinks cans.
But health experts fear this will hinder efforts to reduce obesity levels and tackle diabetes.
Shona Hilton, professor of public health policy at the University of Glasgow, Professor Alec Morton, founding fellow of the Centre for Health Policy at University of Strathclyde and Dr
Simon Hunter from the same institution, raised their concerns along with Dr Joan Costa-i-Font and Professor Alistair McGuire, both from the department of health policy at London School of Economics.
In a letter to The Herald newspaper, they said the Scottish Government had set "ambitious plans to halve childhood obesity by 2030", with Holyrood ministers also likely to restrict advertising of fatty, sugary foods.
They noted the 20p charge planned for the deposit return scheme would apply to drinks containers "regardless of size - from 330ml servings of lemonade to 750 ml bottles of sparkling juice and two-litre containers of cola".
The experts warned: "For smaller servings of less than half a litre a 20p deposit represents a huge percentage increase on the price charged to the consumer.
"For larger containers the increase is less significant.
"A flat deposit fee thus risks incentivising consumers to purchase larger containers of sugary drinks.
"This in turn could severely hamper the Scottish Government's efforts to meet its targets on childhood obesity."
They added: "If we are to make meaningful progress in the fight against obesity and diabetes, we need a deposit return scheme alive to the realities of public health in Scotland today.
"This means a graduated deposit which goes up in proportion to the size of the container."
A Scottish Government spokesman said: "Tackling obesity is a priority for the Scottish Government and last year we published a comprehensive plan which sets out a wide range of actions to support people to eat well and have a healthy weight.
"This includes our ambition to halve childhood obesity by 2030, our investment of £42 million in weight management services and proposals to restrict in-store promotion and marketing of foods high in fat, sugar or salt with little or no nutritional benefit.
"We were the first part of the UK to commit to a deposit return scheme which is ambitious in scale and scope and gives the people of Scotland a clear and straightforward way to do their bit for the environment."
John Mayhew, director of the Association for the Protection of Rural Scotland, which has campaigned for a deposit return scheme since 2015, said the calls from health experts "seems to be based on a misunderstanding of how deposit return works".
He said: "A deposit does not add to the cost of a drink - it is fully refunded when you take an empty can or bottle back.
"What's more, some deposit return systems internationally use a flat deposit rate and others use different levels for containers of different sizes.
"No evidence has been presented that suggests either approach has any impact on obesity."