Thailand's Princess Bajrakitiyabha, the eldest child of King Maha Vajiralongkorn, remains unconscious in hospital after collapsing in mid-December, the palace has said.

Doctors of the 44-year-old princess believe she developed an infection caused by mycoplasma bacteria and is suffering from heart inflammation resulting in severe cardiac arrhythmia, according to a statement of the Royal Household Bureau issued on Sunday.

The princess is on medicine to restore her heart, lung and kidney functions while her condition is being closely monitored, according to the statement.

Princess Bajrakitiyabha is one of three of King Vajiralongkorn's children who have formal titles, but the 70-year-old king has yet to appoint a royal heir since he assumed the throne in 2016.

The princess has served as the Thai ambassador to Austria, Slovakia and Slovenia, and has also engaged in work with the United Nations on protecting women.

She currently serves as a public prosecutor and is involved in humanitarian assistance projects, especially in promoting women's rights in the criminal justice system.

==Kyodo