"It was a downhill road and the brakes failed, and the driver lost control of the vehicle before it overturned," said Colonel Sophon Phramaneehe, adding that those who died on Wednesday were adults on a study trip.
There were 49 people on the bus, all Thai, including the driver, the police official told Reuters.
Social media posts showed rescue and medical workers at the scene in Prachinburi province, 150km east of the capital Bangkok, helping victims near the bus with its undercarriage exposed.
Thai Prime Minister Paetongtarn Shinawatra expressed her condolences to the victims' families and said an investigation was taking place.
"If it is found that there is a violation of the use of vehicles that do not meet the standards or are involved in reckless use of vehicles, legal action will be taken," she said in a post on X.
"Inspection of vehicles must be safe and pass the specified standards before they are put into use to prevent accidents and reduce losses like this again," she said.
Road accidents and fatalities are common in Thailand due to weak enforcement of vehicle safety standards and poorly maintained roads.Â
The Southeast Asian nation ranked ninth out of 175 World Health Organisation member countries for road traffic deaths, according to its 2023 report.
In 2024, a school bus caught fire due to a gas cylinder leak, killing 23 people, including 16 students.
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