Thailand’s divisive former prime minister Thaksin Shinawatra was transferred from prison to a hospital early on Wednesday, less than a day after he returned from an extended exile and started serving an eight-year sentence.

The 74-year-old had returned to Thailand after 15 years abroad on the same day a party linked to him won a parliamentary vote to form a new government.

He was then sent to prison to serve sentences for several criminal convictions made in absentia that he had decried as politically motivated.

The prison reported that he had high blood pressure and low oxygen, he could not sleep and felt tightness in his chest, according to a statement from Sitthi Sutivong, deputy director-general of the Corrections Department.

Doctors at the prison’s hospital said he should be transferred to prevent life-threatening risks, the statement said.

Corrections officials had previously said he was considered vulnerable due to his age and chronic heart and lung conditions, high blood pressure, and back problems.

Hours after Thaksin’s return to Thailand, Pheu Thai party candidate Srettha Thavisin secured enough votes in Parliament to become prime minister, ending more than three months of suspense, legal wrangling and horse trading that followed May elections.

The party had entered a coalition with military parties linked to a coup that removed it from power in 2014, and excluded the progressive Move Forward party which won the most votes in the polls.

Mr Srettha received a royal letter of endorsement officially appointing him as prime minister on Wednesday, with several key members of Pheu Thai and other coalition parties also attending the ceremony.

He gave a speech thanking the king, the Thai people and officials. He pledged to work to “bring the country forward” and solve economic issues and inequality.

He said he was confident his four-year term “will be four years of change”.

Pheu Thai is the latest in a string of parties affiliated with Thaksin, who was ousted in a 2006 military coup.

That coup triggered nearly two decades of deep political divisions which pitted a mostly poor, rural majority in the north that supports Thaksin against royalists, the military and their urban backers.

A Pheu Thai government led by Thaksin’s sister, Yingluck Shinawatra, was ousted in the 2014 coup by then-army chief Prayuth Chan-ocha, who is now the outgoing prime minister.

It had been widely speculated that Thaksin returned out of hope that a friendly government would reduce his sentence, although he has said his decision had nothing to do with the Pheu Thai party’s bid for power and that he was ready to follow the legal process.

The outgoing government has said he can request a royal pardon like any other inmate.