Police have pressed four charges against the driver of the bus that burst into flames, killing a least 20 students and three teachers in Pathum Thani on Tuesday.

The driver, Samarn Chankut, fled the scene of Tuesday's tragedy.

He turned himself in to police on Tuesday night.

Mr Samarn told police he was in deep shock when he left the scene of the fire and fled to a relative’s house in Ang Thong.

His bus crashed and burst into flames on Vibhavadi Rangsit road, near the Zeer Rangsit shopping centre, around noon.

Mr Samarn told police they were travelling at 70-80 kilometres per hour when he felt the bus suddenly lose balance.

He lost control of the vehicle, which crashed into a Mercedes Benz before hitting the median barrier. Flames then spread rapidly through the bus.

Police initially pressed four charges against Mr Samarn - reckless driving causing danger to people or property, reckless driving causing death, careless driving causing physical or mental harm to other people, and failing to provide assistance after an accident.

The bus was carrying 38 students and six teachers from Wat Khao Praya Sangkharam school in Lan Sak district of Uthai Thani province.

Twenty students and three teachers on the field trip were confirmed killed by the fire. 

Three injured students are in critical condition.