The influential founder of Thailand's populist Pheu Thai Party was transferred to a police hospital overnight, 15 hours after a vaunted homecoming from 15 years abroad to avoid jail that coincided with his political ally being elected prime minister in a parliamentary vote.
The Corrections Department said the 74-year-old Thaksin was transferred in the early hours of Wednesday after being unable to sleep and experiencing chest tightness and high blood pressure on his first night in prison.
"Physicians agreed that to prevent dangerous risks to his life, he be transferred to the police hospital," it said.
Soponrat Singhajaru, a senior doctor, confirmed his condition had improved since he was admitted.
A representative for Thaksin declined to comment.
Thaskin, a hugely popular prime minister among many voters for pro-poor policies, was ousted in a 2006 military coup and was in self-exile two years later when he was sentenced to jail for graft and abuse of power.
His return is the latest twist in chaotic few months in Thai politics during which the progressive Move Forward party triumphed in a May election but ended up in opposition, and the populist heavyweight Pheu Thai, twice toppled by the military, agreed to form a government with army-backed parties.
Thaksin returned on a private jet to cheering crowds on Tuesday before being whisked away to a court and then to jail in dramatic scenes that stole the spotlight from fellow tycoon Srettha Thavisin, who was chosen as prime minister hours later.
The unimpeded return of Thailand's most divisive politician and Srettha's surprisingly smooth ascent to the top job will add to speculation Thaksin has struck a deal with longtime enemies in the military and conservative establishment for his safe return and, possibly, an early release from jail.
Thaksin and Pheu Thai have denied that.
Srettha, a United States-educated property mogul, has been endorsed as premier by the king and is expected to form an 11-party coalition.
Election winner Move Forward declined to back Srettha, calling his alliance with unpopular pro-military parties a betrayal of the will of the people.
