But Mr. Rajapaksa’s latest populist gesture — ordering a 40 percent reduction in a tax on sugary drinks — has incensed experts who say the policy was mitigating sugar’s adverse effects on public health.

The original tax, introduced in November 2017, was 50 Sri Lankan cents for every gram of sugar in sweetened beverages. (One United States cent is worth about 178 Sri Lankan cents.) Such taxes, along with outright bans on junk food or soft drinks, are often seen by health experts around the world as a sensible way of reducing the incidence of a number of noncommunicable diseases.

Such diseases account for an estimated 83 percent of all deaths in Sri Lanka, according to the World Health Organization. Diabetes alone accounts for 9 percent of deaths in the country, and the incidence of both obesity and high blood pressure there has been rising steadily over roughly the last two decades.

But over the weekend, the Finance Ministry ordered the sweetened beverage tax immediately reduced to 30 Sri Lankan cents per gram of sugar from 50 Sri Lankan cents, the local news site EconomyNext reported. The ministry received the order after Mr. Rajapaksa met with representatives from companies that make sweets and sugary drinks, the site said.

“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.”

Mr. Sirisena is the co-chairman of a commission on noncommunicable diseases at the World Health Organization. In a speech for World Diabetes Day last year, he called on Nestlé to reduce the sugar content in Milo, one of its popular drinks, if it wanted to avoid tougher policies on sugary products.

Consumption of sugar-sweetened beverages is highest in Australia, North America, Latin America and Western Europe, and lowest in the Asia-Pacific region, the Asian Development Bank Institute said in a report this year.

But sales of sugar-sweetened beverages are increasing in many low- and middle-income countries even as they decrease in high-income countries, the report said. It also noted that the percentage of overweight and obese people in Sri Lanka rose almost 36 percent from 1990 to 2013, the second-fastest increase in South Asia after Nepal.