“This is highhanded and wrong, and we do not like it,” Dr. Mahen Wijesuriya, an expert on diabetes and the executive director of the Non Communicable Diseases Alliance Lanka, a nonprofit group in the city of Rajagiriya, said of the revised beverage tax.

Though Mr. Rajapaksa lacks parliamentary support, he can still legally tweak some tax rules outside the normal parliamentary process, EconomyNext reported.

But Dr. Wijesuriya said the decision to reduce the sweetened beverage tax had been made without consulting the country’s medical community. He said the new rule was likely to make sugary drinks more prevalent and contribute to a further spike in the incidence of obesity and diabetes.

Dr. Wijesuriya added that Mr. Sirisena, a former health minister, once backed the 50-cent tax and should have pushed back on any effort by his own cabinet to weaken it.

“If he stood by his convictions, he should have avoided this issue by getting the prime minister not to do this kind of thing,” Dr. Wijesuriya said in a telephone interview.

Representatives for the offices of the Sri Lankan president and the prime minister did not immediately respond to requests for comment on Monday.

Samantha Mendis-Wedage, a spokeswoman for Nestlé Lanka, declined to comment on the sweetened beverage tax. “As always, we will continue to comply with the rules and regulations of the country,” she said in an email. “We are committed to reduce sugar, salt and fat in our products and our work in that direction will continue.”