More than a dozen health organisations have written to Downing Street just weeks before David Cameron launches his childhood obesity strategy

In January, David Cameron said he would not rule out a tax on sugary drinks. Photograph: Lewis Whyld/PA

Leading health groups have called on the government to impose a 20% tax on all sugar-sweetened soft drinks to tackle obesity, arguing that current policies are ineffective.

More than a dozen health organisations from the British Medical Association to the Royal Society for Public Health have written to Downing Street just weeks before David Cameron launches his childhood obesity strategy. The prime minister has not ruled out a sugar tax, after intense public pressure.

Cameron has found himself increasingly isolated as Public Health England, the chief medical officer, the Commons select committee on health, and campaigners including Jamie Oliver have pushed for the introduction of a sugary drinks tax.

In a parliamentary debate on childhood obesity on Thursday, the Tory MP Dr Sarah Wollaston, who chairs the health committee, again called for a sugary drinks tax as well as action on promotional campaigns.

Wollaston and others have pointed to Mexico’s success in reducing consumption of sugary drinks after a sugar tax. A 10% tax on Coca-Cola and other sugar-sweetened drinks in Mexico was implemented on 1 January 2014 after a battle with the beverage industry. An evaluation of the tax by the Mexican National Institute of Public Health and the University of North Carolina said the tax reduced purchases by an average of 6% across 2014, and by as much as 12% in the last part of the year.

“Unequivocal evidence from other countries has shown that a sugar tax duty on soft drinks will reduce sales of sugar-sweetened soft drinks, particularly among the more socially deprived who are more likely to develop obesity and type 2 diabetes,” said the health groups.

“The tax will encourage behaviour change for those who drink sugar-sweetened soft drinks to either change to an artificially sweetened drink (which is lower in calories than sugar-sweetened) or to drink water – an even better option.”

Speaking earlier in January, Cameron said the government would not rule out introducing a tax on sugary drinks to tackle what he called Britain’s obesity crisis. “Of course it would be far better if we could make progress on all these issues without having to resort to taxes,” he said. “But what matters is that we do make progress.”

The NHS is to impose its own sugar tax in hospitals and health centres in England, with a 20% tax on all sugary drinks and foods in NHS cafes by 2020. The annual cost to the NHS of obesity alone is estimated at £5.1bn. Diabetes UK estimates that the NHS is already spending about £10bn a year on diabetes. The medical groups say the costs are not sustainable and will bankrupt the NHS.

Separately, the public accounts committee warned in a report that the number of diabetes patients experiencing complications continued to increase, while diabetes specialist staffing levels in hospitals were not keeping pace with the increasing percentage of beds occupied by sufferers of the disease.

The committee said while progress had been made since it last examined diabetes services, there remained “unacceptable variations in the take-up of education programmes, delivery of recommended care processes, achievement of treatment standards and in outcomes for diabetes patients”.