BANGKOK, Dec 15 (AFP) - The eldest child of Thai King Maha Vajiralongkorn has been hospitalised due to a heart problem and her condition has stabilised to a certain level, the royal palace said on Thursday.

Princess Bajrakitiyabha, 44, was taken ill after losing consciousness early on Wednesday in northeastern Nakhon Ratchasima province and was treated at a local hospital.

She was flown by helicopter to Bangkok after her condition stabilised at a certain level, the palace said in a statement, without elaborating.

The princess is one of three children of King Vajiralongkorn who have formal titles, making her eligible for the throne under a 1924 Palace Law of Succession and the country's constitution.

King Vajiralongkorn has yet to formally designate an heir since becoming king in 2016 and there has been no official discussion on the prospect of the princess taking the throne.

The princess, who has done work with the United Nations and has served as Thailand's ambassador to Austria, Slovenia and Slovakia, was attending a working dog championship organised by the army when she lost consciousness, the palace said.

She was being treated at Bangkok's Chulalongkorn hospital on Thursday where she was undergoing treatment and checks on her condition, it said.

While the palace succession law stipulates that the heir to the throne should be male, an amendment to the constitution in 1974 allowed for a daughter of the royal line to ascend the throne if a successor had not been named.