The eldest son of Cambodia's long-term ruler has been endorsed as the country's next premier in a formality confirming the transition of power.

On Monday, Cambodia's king issued a decree stating Hun Manet will succeed Hun Sen, who has ruled for 38 years.

Hun Sen announced he would step down just days after the 23 July election, which critics said was not democratic.

His party won all but five seats in parliament, after the main opposition was barred from the vote.

King Norodom Sihamoni issued the royal decree after Hun Sen sent a letter requesting the official note.

Hun Manet, 45, who until recently was the commander of the Royal Cambodian Army, has long been groomed for the leadership role.

His appointment still needs to be confirmed by parliament on 22 August, but it is expected to sail through a National Assembly where 120 of 125 seats are controlled by Hun Sen's Cambodian People's Party.

The appointment follows a dynastic succession plan which had been signposted by Hun Sen for several years.

Hun Manet's new cabinet is expected to include younger faces. Many of the men who rose with Hun Sen through the Khmer Rouge revolution and civil war of the 1970s and 80s have also stepped down, in some cases handing their jobs to their own children.

The transition was first flagged in 2021, but until July it was unclear when it would occur.

However, Hun Sen announced that he would step down just three days after the election. He is one of the world's longest-serving leaders, having ruled the South East Asian nation of 16 million people for nearly four decades.

He said at the time he was resigning to ensure stability in Cambodia.

Hun Sen will however retain leadership of the ruling Cambodian People's Party - a position political analysts say still gives him ultimate control.

His rule has become increasingly authoritarian since coming to power in 1985, and he has silenced opponents by either jailing or exiling them.

On the weekend, in addition to marking his 71st birthday, Hun Sen confirmed the party's landslide victory in the July election - polls which the US, EU and other Western nations have said was neither free nor fair.

Hun Manet celebrated Saturday's election result by posting a photo to Instagram showing his son presenting a bouquet of flowers to Hun Sen with the caption: "Happy birthday to respected and beloved father."

The outgoing leader on Saturday also confirmed Hun Manet's election as an MP for Phnom Penh, removing the remaining procedural obstacle for his ascent to power.

With all meaningful opposition shut down in Cambodia in recent years, Hun Manet will face few threats from outside the ruling party. But maintaining the complex web of alliances his father built with other powerful and wealthy families will be a challenge.

Hun Sen dispensed political and business privileges to potential rivals to keep them happy, and the economy growing. But he also saddled Cambodia with incendiary levels of corruption and inequality which could store up trouble for his inexperienced son.

Compared to his father, some have speculated that Hun Manet, who attended US military academy West Point and the University of Bristol, could lead a less repressive regime and be more receptive to Western calls on human rights.

However, there is little evidence to suggest he will be more open. Hun Sen has also made it clear that he will continue to wield enormous influence over the running of the country for at least for another 10 years.

In a telegram post on Monday, Hun Sen wrote of his son's appointment: "It is not the end yet. I will continue serving in other positions for at least until 2033."