Former prime minister Tanin Kraivixien, 89, has been named the new privy council president, in a temporary appointment pending Crown Prince Maha Vajiralongkorn's accession to the throne.
Mr Tanin, who helmed Thailand from 1976 to 1977, now leads the panel of royal advisers handpicked by King Bhumibol Adulyadej, who died last Thursday after a long illness .
The appointment was disclosed yesterday by Deputy Prime Minister Wissanu Krea-ngam, the legal expert within the military-dominated Cabinet.
Shortly after King Bhumibol's death, the heir apparent had asked for time to mourn with the people before becoming King.
This triggered a legal process requiring then privy council chief Prem Tinsulanonda, 96, to act as Regent.
As the two appointments cannot be held by the same person, Mr Tanin was picked to be acting president.
Prime Minister Prayut Chan-o- cha has stressed that the national legislature will formally invite the Crown Prince to ascend the throne after religious rites for King Bhumibol are over, but has so far not given a date.
Royal funerals can take place months after the death of a monarch, while the government has declared a one-year mourning period.
The country of 68 million people is in deep mourning over the passing of King Bhumibol, who reigned for 70 years amid frequent changes of government brought on by military coups.
While Thailand is a constitutional monarchy, the late King was a unifying figure who in 1992 intervened personally to end a bloody political conflict.
Many aspects of this royal transition remain opaque in the politically divided kingdom, where anyone convicted of lese majeste - insulting or defaming the King, Queen, heir apparent and Regent - could be jailed for up to 15 years on each count.
Mr Tanin is a former Supreme Court judge and staunch anti-communist who became prime minister following a military coup staged in 1976, shortly after left-wing student protesters were massacred in Bangkok.
His own administration was upended by another coup in 1977. King Bhumibol then appointed him to the privy council the same year.
Last year, Mr Tanin wrote a letter to Mr Prayut in which he urged the government to bar corrupt politicians for life and to seize their assets.
According to Thai media reports, he has also supported the idea of building a canal across southern Thailand that would allow international cargo to bypass the Malacca Strait and boost Thailand's standing as a transportation hub.
Meanwhile, the Thai authorities are stepping up surveillance of insults against the monarchy.
Police say there have been 12 lese majeste cases since last Thursday, with four people arrested, and arrest warrants issued for the rest.
The increasing prosecution of lese majeste offences has been accompanied by a spate of vigilantism, where mobs confronted and even assaulted people accused of insulting the monarchy.