Thai King Bhumibol Adulyadej reads a statement convening Parliament in front of 391 newly elected members of parliament at Ananta Samakhom throne in Bangkok in this July 10, 1995 file photo.  [Photo/Agencies]

BANGKOK - Thailand's King Bhumibol Adulyadej, who was the world's longest-reigning monarch, died in hospital on Thursday, the palace said in an announcement.

The palace did not give a reason for his death. He was 88 years old.

"His Majesty has passed away at Siriraj Hospital peacefully," the palace said, adding he died at 15:52 (0852 GMT).

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