Thailand's frail king is expected to leave hospital and travel to his coastal palace on Thursday after almost four years as an in-patient, a government official said.

King Bhumibol Adulyadej, 85, is due to be discharged from Bangkok's Siriraj Hospital, where he has lived since September 2009, said Weera Sriwathanatrakoon, the governor of Prachuap Khiri Khan province where his palace is located.

He will travel to his residence in the seaside town of Hua Hin, about two hours drive south of Bangkok, with 80-year-old Queen Sirikit, who has also been treated in hospital for a year.

"I got reconfirmation from the Royal Household Bureau that their majesties would leave Bangkok at 4:00pm today (0900 GMT)," Weera told AFP.

"I think that more than ten thousand Hua Hin residents will turn out to greet the king because they have been expecting him," he added.

Thai television showed banners lining the Hua Hin streets in preparation for the royal visit, with large posters of the king and queen adorned with offerings.

The palace has not yet confirmed the move and there has been no official explanation for the move.

The king is revered as a near-deity in the Southeast Asian nation, which has been torn by often bloody political divisions in recent years.

The elderly monarch, who with 67 years on the throne is the world's longest-serving royal, has suffered from a range of ailments since being admitted with respiratory problems in 2009.

He suffered a minor brain bleed in July 2012, but has since made several official appearances including meeting Barack Obama during the US president's visit to the country in November.

The queen has largely disappeared from public life since July 2012, when she was admitted to Siriraj Hospital with what doctors termed a slight loss of blood flow to the brain.

Weera said he did not know how long the royal couple would stay at the Hua Hin palace -- which is called Klai Kangwon, or "far from worries" -- a longtime favourite of the royals.

The king has no official political role but called for stability during his birthday celebrations in December.

Thailand has been rocked by sporadic rival street protests for years, with ultra-royalist nationalist "Yellow Shirts" and rural working class "Red Shirts" both taking to the streets.

It is again gearing up for anti-government protests in central Bangkok, with a newly-formed coalition of ultra-royalist groups who despise the Puea Thai ruling party and its exiled figurehead Thaksin Shinawatra vowing to rally from Sunday.