Thai Princess Bajrakitiyabha Mahidol, the king's eldest daughter, has died aged 47, the royal palace announced Friday, more than three years after she was hospitalised following a sudden illness.
She was suffering from an abdominal infection and "her condition continued to worsen", the Bureau of the Royal Household said in a statement, until she "passed away peacefully" on Thursday evening.
She will lie in state at the Grand Palace in Bangkok and her funeral would be held "with the highest honours according to royal tradition", it added.
Known in Thailand as "Princess Bha", the only child from King Maha Vajiralongkorn's first marriage had been in hospital since falling ill suddenly in December 2022.
The Bureau of the Royal Household said in May that her health condition had deteriorated and she was relying on medical devices to support her lung and kidney functions, as well as medication.
Bajrakitiyabha was the only child of King Vajiralongkorn's first marriage to Princess Soamsawali.
A trained prosecutor and diplomat, Bajrakitiyabha was educated in Britain, Thailand and the United States, earning a law degree from Cornell University, and serving for a time as Thailand's ambassador to Austria.
During a 2012 visit to the US law school, the princess remarked about her professional background to a campus audience: "I ask myself now, what am I exactly? A prosecutor? A criminal lawyer? A diplomat? The answer is everything all together. I say I am a hybrid," according to the Cornell Chronicle.
{{/usCountry}}During a 2012 visit to the US law school, the princess remarked about her professional background to a campus audience: "I ask myself now, what am I exactly? A prosecutor? A criminal lawyer? A diplomat? The answer is everything all together. I say I am a hybrid," according to the Cornell Chronicle.
{{/usCountry}}She also held several positions with the United Nations and became an advocate for women's rights, including improved conditions for women in prison.
As princess, Bajrakitiyabha held an important ceremonial role in Thai society -- where the royal family sits at the apex.
She was seen as close to her father, and was appointed to a senior role in his bodyguard command a year before her hospitalisation.
The 73-year-old king, who has seven children from four marriages, has not announced his chosen heir, though succession rules favour men.
Strict rules govern what can and cannot be said about the Thai royal family, who are protected from criticism by lese-majeste laws which carry prison sentences of up to 15 years per charge.