Polarising billionaire Thaksin, the central figure in a tumultuous two-decade battle for power in Thailand, left on his private jet for Dubai late on Thursday, with his family's ruling party Pheu Thai in disarray.
Thaksin's flight out of Thailand came just days before a court ruling scheduled for next week that could see him jailed.
The departure of Thaksin, the driving force behind Pheu Thai, came six days after a court sacked his daughter and protege, Paetongtarn Shinawatra, as prime minister for an ethics violation, triggering a scramble for power and a bold offensive by a renegade party to form its own government.
Pheu Thai, the populist political juggernaut that won five of the past six elections, has fought desperately to thwart the challenge of former alliance partner Bhumjaithai, which has won the backing of the biggest force in parliament with a pledge to call a new election within four months.
The turmoil has put Bhumjaithai leader Anutin Charnvirakul in pole position before Friday's vote, where he needs the support of more than half of the lower house to become prime minister.
His coalition has 146 MPs, and with the People's Party opting to stay in the opposition but guaranteeing him its 143 votes, Anutin could comfortably win the required 247 votes.
After a failed attempt to dissolve the house to stymie Anutin, Pheu Thai made another last-ditch attempt to undermine his alliance on Thursday, announcing it would nominate 77-year-old former attorney-general Chaikasem Nitisiri to contest the prime ministerial vote, with a promise to call a snap election immediately if elected.
Both candidates were nominated by their parties and endorsed by parliament before the vote, which is expected to take place after a debate among MPs.
But with the sudden departure of 76-year-old power-broker Thaksin amid a crisis in his once-dominant party, the chances of political unknown Chaikasem succeeding look increasingly slim.
In an overnight post on X, Thaksin said he had arrived for a medical check-up in Dubai, where he spent most of his 15 years in self-imposed exile to avoid a jail term for abuse of power and conflicts of interest while he was prime minister from 2001 to 2006.
He said he would return by Monday.
Thaksin made a vaunted homecoming before cheering crowds in 2023 to serve his eight-year sentence, but on his first night in prison, he was transferred to the VIP wing of a hospital on medical grounds.
The tycoon had his sentence commuted to a year by the king and was released on parole after six months in detention.
The Supreme Court will decide on Tuesday if his hospital stint counts as time served. If not, it could send him back to jail.
Wanwichit Boonprong, a political science lecturer at Rangsit University, said Anutin had outmanoeuvred Thaksin's Pheu Thai by making a pact with the opposition.
"I'm quite confident that Anutin will be elected as the next prime minister," he said.
"Pheu Thai's tactics are like the final show," he said.
"Pheu Thai has completely closed the curtain."
