The Thai king's eldest daughter has been airlifted to a Bangkok hospital for treatment after losing consciousness due to a heart condition, the palace said today.

Princess Bajrakitiyabha Mahidol intially fell ill Wednesday evening during a military dog training session at Nakhon Ratchasima north of the capital, the palace said.

Known in Thailand as "Princess Bha" the 44-year-old is the eldest daughter of King Maha Vajiralongkorn and the only child from his first marriage.

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.