DOCTORS' leaders have joined in the call for a tax on sugar, saying a 20p levy on sugary drinks would be a "useful first step" towards the long-term goal of reducing obesity.

The British Medical Association (BMA) said imposing the tax could reduce the prevalence of obesity in the UK by around 180,000 people.

Yet critics have argued the evidence for the levy is not as "compelling" as the BMA suggests and a so-called 'Irn-Bru' tax was not the way forward.

Recent figures show that one in three adults in Scotland are thought to be obese, while one in three children are also at risk, and cost economy £145 every second and drain the NHS of up to £600 million annually.

The report's author, BMA's board of science chairwoman Professor Sheila Hollins, said: "While sugar-sweetened drinks are very high in calories they are of limited nutritional value and when people in the UK are already consuming far too much sugar, we are increasingly concerned about how they contribute towards conditions like diabetes.

"We know from experiences in other countries that taxation on unhealthy food and drinks can improve health outcomes, and the strongest evidence of effectiveness is for a tax on sugar-sweetened beverages.

If a tax of at least 20 per cent is introduced, it could reduce the prevalence of obesity in the UK by around 180,000 people."

Previous reports have shown that the gap between the price of healthy and unhealthy foods is widening, with the price of unhealthy foods being shown to decrease over time

The BMA's report suggests that imposing a minimum 20p tax on all non-alcoholic water based beverages with added sugar -- including sugar-sweetened soft drinks, energy drinks, fruit drink, sports drinks and fruit-juice concentrates -- could subsidise the sale of fruit and vegetables.

Professor Hollins added: "We know that the majority of the UK population, particularly low income households, are not consuming enough fruit and vegetables, so financial measures should also be considered to subsidise their price, which has risen by 30% since 2008.

However Gavin Partington, Director General of the British Soft Drinks Association, said evidence from other countries had shown this type of tax doesn't work.

He said: "The soft drinks industry recognises it has a role to play in supporting public health objectives and welcomes steps to encourage a balanced diet and active lifestyle but targeting single ingredients or products is misguided and unlikely to prove effective."

Meanwhile Scottish Conservative health spokesman Jackson Carlaw said "the evidence is nothing like as compelling as the BMA suggest." adding, "an 'Irn Bru' tax is not the way forward".

"This is not to diminish the hugely serious and ever growing problem of obesity," he said. "A fundamental change in behaviours is required but simply crying out for more for legislative taxes will win neither hearts nor heads and will make the educational challenge even more problematic."

Jenny Marra, Shadow Health Minister for Scottish Labour, said a more robust study of the impact of the tax would be needed before taking a firm view on it.

She said: "We recognise obesity is a significant problem and there are clearly lessons we can learn from other countries. The SNP Government should be looking at all options."

The BMA's report also said it would also like to see a ban on all marketing of unhealthy food and drink products to children and young people, adding that a "particular area of concern is the proliferation of marketing online and via social media".

A Scottish Government spokesman said it would consider the evidence first before considering taxes to help improve diet, adding: "We need to be certain that the impacts on our health are positive without increasing inequalities or disproportionately affecting the food and drink industry€Ž."