Thailand's junta leader Prayut Chan-o-cha attends the Palang Pracharath Party's party campaign rally in central Bangkok. File picture: Soe Zeya Tun/Reuters

Bangkok - Thai junta leader Prayut Chan-o-cha was officially endorsed by King Maha Vajiralongkorn as prime minister on Tuesday, almost a week after he won an overwhelming victory in parliament.

"My family and I will forever owe a debt of gratitude to His Majesty's kindness," Prayut said in a speech at Government House.

"I affirm that I will dedicate myself to public service with honesty and integrity," he added. 

The 65-year-old retired general seized power in a May 2014 coup and made himself prime minister the same year. 

In a bid to give himself legitimacy to continue to rule, Prayut's regime allowed an election to be held but rigged the rules in its own favour, including hand-picking all 250 senators and granting them the right to vote for the prime minister alongside the elected lower house.

The Senate's votes, together with the votes from the pro-military Palang Pracharat Party and its allies, cemented Prayut's victory with  500 votes, making up two-thirds of the 750-member parliament when it convened for the first time since the March election last week.

Prayut's rule will transition from a military dictatorship to semi-democracy. 

Although the king's endorsement on Tuesday is a matter of formality, analysts say that without the king's backing, Prayut would not have all the support required to continue his rule. 

It is not immediately known when a new government will be formed and take office. 

The king was not physically present to give Prayut his support. He spends the majority of his time in Germany, where he has a private villa.