Prince Bhisadej Rajani died on Saturday at Siriraj Hospital aged 100, according to the Royal World Thailand Facebook page.
A bathing ceremony will take place on Sunday at 5pm at Wat Benchamabophit. His funeral rites will take place over seven days.
He was one of the most respected royal figures for contributions made since the reign of King Rama IX. Bhisadej, the last surviving member of the Viceroy line, was the first male member of the royal family in decades to reach the age of 100.
Born Jan 20, 1922, he was the son of Prince Rajani Chamcharas, The Prince Bidyalongkorn. Prince Bhisadej was the grandson of the last Viceroy of Siam, Prince Bovorn Vichaicharn.
His mother was Princess Barabimalabarna Varavarn, daughter of Prince Voravarnakorn, The Prince Naradhip Prabanbongse.
He was the great-grandson of both King Mongkut (Rama IV) and his brother Vice-King Pinklao. He has an elder sister, Princess Vibhavadi Rangsit.
During his younger years, he enrolled in Debsirin School and Vajiravudh College before moving to Dulwich College in London.
He soon adapted to life overseas, particularly during World War II, when he served as a soldier, villager and spy for the British army.
When Thailand joined Japan in the Axis, Thai citizens were met with difficulties, especially those who lived in Allied countries like the UK or the US.
A number of Thais living in the UK became pro-Ally and joined the British Army.
The Free Thai Movement was organised later to resist the Axis powers in Thailand, and Prince Bhisadej recruited others to join the British Army.
He was one of the members of an underground national movement who arrived in Thailand as villagers. They were an important source of military intelligence for the Allies in the region.
After Japan surrendered in 1945, the Free Thai Movement helped Thailand gain recognition for its neutrality in conflicts.
Prince Bhisadej received the Agricultural Philosopher Awards 2011.
