King Vajiralongkorn ascended the throne on December 1 after the death of his father, King Bhumibol Adulyadej, on October 13.

"This is the first opportunity since his majesty's ascension to show his mercy," the Royal Gazette said in a statement, announcing that 150,000 inmates could be eligible for release or to have sentences cut, under the pardon.

Officials do not have a figure for the number of inmates who will benefit from the pardon, with decisions to be made by different prisons depending on factors including the inmate's age, how much of the sentence has been served and behaviour.

Prisoners jailed for insulting the monarchy and drug offences will be eligible, said Kobkiat Kasivivat, director general of the Department of Corrections.

"Prisoners convicted of 112 and prisoners convicted of drug offences will be included for consideration for release or commuting of sentence," Kobkiat told Reuters, referring to the royal insult law by its article number in the criminal code.

The government has not released a figure for the number of people who are in jail for royal insult but there have been more than 80 prosecutions under the law since mid-2014, according to figures from the legal monitoring group iLaw.

"The inmates will be looked at on a case by case basis at each individual corrections facility," Chanchao Chaiyanukit, the permanent secretary at the Ministry of Justice, told Reuters.

Those convicted of murder and rape will not be eligible for release or to have sentences cut, officials said.

Thailand's prison population has soared in recent years, largely because of tough drug laws.

Corrections Department figures for July showed a prison population of 321,347 in Thailand, with about 70 percent jailed for drugs offences.