Thai King Bhumibol Adulyadej has died at the age of 88, the royal palace announced Thursday. He died at Siriraj Hospital in Bangkok.

The world’s longest reigning monarch was admitted to the hospital Monday and was on a ventilator fighting a new infection. On Wednesday, the palace had said that his health had not stabilized. Thailand, which was rocked by a military coup in 2014, it's feared could face more political uncertainty with the king's death as the monarch was seen as an uniting force in a country suffering from long-running strife.

"The relationship between Thais and the King is deep, more than one can actually begin to explain," Col. Winthai Suvaree, a spokesman for the royalist junta, had told Reuters earlier this year. "He is a father to the land."

Last December, the king made a rare public appearance at a ceremony to swear in new judges. At the time, he was seen sitting on a gold throne during the swearing-in ceremony at a Bangkok hospital where he was admitted for various health conditions, including a lung infection and hydrocephalus, a build-up of cerebrospinal fluid that surrounds the brain.

Bhumibol was born in 1927 in Cambridge, Massachusetts, when his father Prince Mahidol Adulyadej was a student at Harvard University. Bhumibol was the ninth monarch of the Chakri Dynasty and ascended to the throne on June 9, 1946, after his elder brother King Ananda Mahidol died. He was officially crowned on May 5, 1950.

He could be succeeded by one of his children. Crown Prince Maha Vajiralongkorn is widely disliked by the population because he is considered unpredictable and unpolished, but he seems to have the backing of the nation's ruling military junta. The crown prince’s sister, Princess Sirindhorn, is more popular with citizens and could possibly take the throne instead.

His 63-year-old son Crown Prince Maha Vajiralongkorn is expected to become Thailand's new king.