The crisis erupted last month when King Tupou VI suddenly withdrew his “confidence and consent” to the three ministries.
Some of Tonga’s ministers have announced their resignations after months of a constitutional crisis with the Pacific island nation’s powerful king.
Prime Minister Shaosi Sobaleni announced in parliament on Thursday that he would resign as defense minister, while one of his key allies, Fekitamoeloa Utoikamanu, accepted King Tupou VI’s demands and resigned as minister of foreign affairs and tourism.
Radio New Zealand reported that Mr Sobaleni recommended that Crown Prince Tupotoa Ulukalala, a senior foreign ministry official, be appointed to replace him as minister of defense and foreign affairs.
Tonga has had a constitutional democracy since the late 19th century, and the monarchy relinquished more powers in the 2010 democratic reforms.
But earlier this year, the king abruptly withdrew his “confidence and consent” to appointments to three key ministries, without explaining why.
Malakai Koloamatangi, a political scientist at the University of Fiji, told AFP news agency that the crisis should be seen as part of long-term irreconcilable tensions “between the position of the king and the parliamentary executive.”
Initially, the prime minister refused to accede to the king’s request, with legal advice from Tonga’s attorney general arguing that the king’s move was unconstitutional.
The prime minister’s attitude changed after a heated debate in parliament and after he denied the allegation of insulting the king.
A formal announcement on the cabinet reshuffle is expected later on Thursday.
Tonga is an archipelago with approximately 100,000 people.
The United States opened an embassy in Nuku’alofa last May as competition with China for influence in the Pacific Islands intensifies.
