Thailand's Supreme Court on Tuesday ruled billionaire Thaksin Shinawatra must serve one year in jail because his detention in a VIP wing of a hospital in lieu of prison was unlawful, in another major blow for a powerful family that has dominated politics for two decades.

The court has ordered that Thaksin, a former prime minister, be taken into custody.

His daughter, former prime minister Paetongtarn, said Thaksin remained "in good spirits" despite the sentence.

"My father remains a spiritual leader – whether through his past political role, his contributions to the country, or his sincere intention to be part of improving the lives of the people," she said.

The five-judge bench said the responsibility for Thaksin's lengthy stay in hospital did not solely lie with the doctors and that the polarising billionaire intentionally prolonged his hospital stay.

"Sending him to hospital was not legal, the defendant knows his sickness was not an urgent matter, and staying in hospital cannot count as a prison term," said the ruling read out by a judge.

The 76-year-old power-broker is experiencing a period of political reckoning after his daughter and protégé Paetongtarn Shinawatra was sacked as prime minister by a court 11 days ago – the sixth premier from or backed by the Shinawatra family to be removed by the judiciary or military.

On return from 15 years of self-imposed exile in 2023, Thaksin spent only a few hours in prison before being transferred to hospital complaining of heart trouble and chest pains, prompting widespread scepticism and public outrage.

His eight-year sentence for conflicts of interest and abuse of power was commuted to one year by the king and Thaksin was released on parole after just six months, the entirety of which he had spent in the VIP wing of a hospital.

Following Paetongtarn's sacking last month, days of chaos ensued before her government fell on Friday, outmanoeuvred by challenger Anutin Charnvirakul, who was elected premier by parliament in a humiliating defeat for Thaksin's once unstoppable Pheu Thai party. (Reuters/AFP)