The 79-year-old sultan, one of the world’s longest-reigning monarchs, will be limiting his presence at national and official ceremonies during the recovery process for rest and physiotherapy sessions, the office said in a statement on Tuesday.

“However, His Majesty will continue to carry out official duties during the period,” it said, adding the surgery would take place in Brunei on Sunday.

The sultan, who ascended the throne in 1967, holds multiple roles in Brunei, serving also as its prime minister, armed forces commander, finance minister, foreign minister and defence minister.

Once the world’s wealthiest man, the sultan remains hugely popular in the tiny, oil-rich nation on the island of Borneo, despite holding a firm grip on power in the country of nearly half a million people.

He was briefly hospitalised in Malaysia in May last year due to fatigue during a summit of Southeast Asian leaders in Kuala Lumpur.