Bangkok (AFP) – Thailand's former defence chief is set to be appointed acting prime minister on Thursday, capping a colourful career for the political heavyweight once nicknamed "Big Comrade".

Phumtham Wechayachai earned his moniker over links in his youth to a 1970s student movement that rallied against the architect of a military coup, before their protests were violently crushed.

He fled to the jungle where communist guerrillas were plotting uprisings against the nation's military, and recently he has been questioned over his associations.

But the 71-year-old has successfully transitioned into the limelight from a business role in the empire of Thaksin Shinawatra, the founding force of a dynasty which has dominated Thai politics for decades.

Phumtham has held the defence and commerce portfolios, and had a previous spell as acting prime minister after a crisis engulfed the top office last year.

On Thursday he is due to be sworn in as deputy prime minister and interior minister -- making him acting premier again, after Thaksin's daughter Paetongtarn Shinawatra was suspended from her role.

Born in the suburbs of Bangkok, Phumtham was nicknamed "Auan", meaning "Chubby", by his parents.

He earned a political science degree from a top Thai university and joined the student movement that took to the streets in 1976, opposing the return of military dictator Thanom Kittikachorn.

His childhood nickname belied his slim-faced appearance in a black-and-white photo of the protests, showing him brandishing speech papers with a microphone in hand.

The uprising ended in a bloody crackdown known as the "Thammasat Massacre" that killed at least 40 students and remains today one of the country's most notorious instances of protest bloodshed.

Unofficial estimates suggest the death toll could have been as high as 500, because live ammunition was used to quell the unrest.

Students from Thailand's elite universities fled into the jungle to join guerilla movements.

When Phumtham became defence minister last year he faced a grilling by the conservative and pro-military establishment who accused him of being a card-carrying communist.

"I went to escape the violence," he insisted. "It was not only me, there were other students too."

Despite his protestations, his links to the movement earned him a second alias: "Big Comrade".

Phumtham's reputation has softened since his firebrand formative years, and he is now known as a composed and diplomatic operator.

He will step into the acting prime minister role after the Constitutional Court suspended Paetongtarn pending an ethics probe which could take months.

In the brief interim between the court decision and Phumtham being sworn in as part of a cabinet reshuffle, transport minister Suriya Jungrungreangkit has been acting premier.

Phumtham's rise has mirrored that of Thaksin, whose dynastic parties have been jousting with the country's pro-monarchy, pro-military establishment since the early 2000s.

In the 1990s Phumtham was employed by the Thaksin-founded telecom giant Shin Corp, before entering politics full-time in 2001.

He served as deputy secretary-general of the Thai Rak Thai (Thais Love Thais) party, founded by Thaksin, and was appointed deputy transport minister in 2005.

After Thaksin was ousted in a coup, the party was dissolved and Phumtham was slapped with a five-year ban from politics.

But the movement remained a potent force, with Thaksin's sister and brother-in-law both having stints as prime minister.

Paetongtarn was appointed in August, with the backing of the family's Pheu Thai party.

Phumtham, considered Thaksin's confidant, appeared by Paetongtarn's side as she gave her first press conference as leader.

Although he will be stepping into her shoes, he has signalled he remains loyal to the Shinawatra dynasty and told journalists he believes she will "survive the probe".