Princess Bajrakitiyabha Mahidol has been treated since December 2022 for heart problems. She has not been conscious since the hospital admission.
Now her condition has worsened. The 46-year-old princess has been affected by a serious blood infection, so-called bacteremia, states the Thai royal house according to news agencies.
Friday's update is the first about the princess in over two years.
Bajrakitiyabha has been seen by some analysts as a potential successor to the 73-year-old King Maha Vajiralongkorn, who has not yet appointed a successor. She would then become the first female monarch in Thailand.
Vajiralongkorn has seven children from four different marriages.
