The palace official statement suggests that Princess Bajrakitiyabha Mahidol, known in Thailand as "Princess Bha" out running with her dogs on Wednesday evening during a military dog training session in the Khao Yai national park when she collapsed.
The 44-year-old is the eldest daughter of King Maha Vajiralongkorn and the only child from his first marriage. The eldest child of Thai King Maha Vajiralongkorn's condition has stabilised to a certain level, the palace official documents stated.
Journalist Andrew MacGregor Marshall said, CPR was carried out “for more than an hour" but Princess BajrakItiyabha could not be revived. It is claimed she has been put on an oxygen machine, the report said.
Andrew MacGregor Marshall tweeted, “Some sources are saying that despite CPR being carried out for more than an hour, there was no response and she has been put on an ECMO machine, which basically means she is dead but being artificially kept alive."
The kingdom's succession rules favour male heirs however the palace has not formally announced an heir apparent.
Bajrakitiyabha plays an important ceremonial role in Thai society -- where the all powerful royal family sits at the apex, protected from criticism by harsh defamation laws which carry prison sentences of up to 15 years per charge.
She was educated partly in Britain before studying undergraduate degrees in law and international relations at university in Thailand.
She later completed a masters degree and PhD in law at Cornell University in New York.
She was Thailand's envoy to Austria in 2012 and Permanent Representative to the United Nations Office at Vienna.
Bajrakitiyabha has also held positions in the Thai legal system, working as a prosecutor with a focus on countering major drug traffickers.
Last year she took up a chief-of-staff position in the King's close bodyguard command.
