Thailand's Princess Bajrakitiyabha, who has been in a coma for more than three years, has died, the royal household has announced. She was 47.

She collapsed in December 2022 while exercising with her dogs. Her doctors attributed it to a severely irregular heartbeat, caused by a mycoplasma infection in her heart.

With her death, the Thai royal family has lost its most visibly accomplished member, and someone who might have played a pivotal role in an as-yet-unclarified succession.

She was the eldest of King Vajiralongkorn's seven children, born on 7 December 1978 to his first wife and cousin, Princess Soamsawali.

"The medical team provided the closest and most intensive care possible, but her condition continued to decline progressively," the palace said in a statement on Friday morning, adding that she passed away at 19:48 local time (12:48 GMT) the previous day in Chulalongkorn Hospital.

She trained as a lawyer, getting two post-graduate degrees from Cornell University in the US. She worked briefly at the Thai mission to the United Nations in New York before returning to Thailand to work in the Attorney-General's offices in Bangkok and elsewhere in the country.

From 2012 to 2014, she was Thailand's ambassador to Austria, where she built a relationship with the UN Office on Drugs and Crime (UNODC).

She started speaking out on the need for penal reform, with a particular focus on vulnerable women who end up in prison; Thailand has one of the world's highest numbers of female inmates.

Once back in Thailand, she became the UNODC's Ambassador for the Rule of Law in South East Asia and continued to advocate for reform of Thailand's criminal justice system, in which severe sentences are often handed down to people convicted of relatively minor drug possession charges.

In 2021, her father made her a chief of staff in his private bodyguard, giving her the rank of general.

Princess Bajrakitiyabha was also a fitness enthusiast who often took part in long-distance runs.

Her abilities and the trust her father appeared to have in her made her an inevitable topic of speculation about the royal succession.

King Vajiralongkorn, who is 73 years old, has not yet named an heir. Thai custom dictates that the heir must be male, but a 1974 amendment to the constitution allows a female to take the throne.

The king has five sons, but four of them, born of his second marriage, were disowned in 1996 and have lived with their mother in the US since then.

His fifth son, Dipangkorn, by his third wife, is the presumed heir, although questions have been raised about his ability to serve as monarch in a country where the royal institution wields so much influence.

For many Thai royalists, Princess Bajrakitiyabha seemed the most promising figure to succeed her father, either as queen or as a regent to help Prince Dipangkorn.

Her death leaves the question of the succession in Thailand unanswered, and the severity of the country's lese majeste law rules out any public discussion of it.