The Thai king's eldest daughter remained in hospital on Monday receiving support for her heart, lungs and kidney, according to a palace statement after she collapsed last week.
Princess Bajrakitiyabha Mahidol initially fell ill Wednesday evening during a military dog training session at Nakhon Ratchasima, north of the capital Bangkok.
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 favor male heirs; however, the palace has not formally announced an heir apparent.
Following the princess' collapse, she was flown to Bangkok where she continues to receive intensive medical care under close observation.
In a statement issued Monday morning, the palace said her condition was "stable at one level", without elaborating.
"Her royal highness's heartbeat is controlled by medicine, the statement said, adding that the systole — part of the process by which the heart beats — "does not go well".
"The medical team has offered her royal highness medicine and equipment to support the work of her royal highness's heart, lung and kidney," the statement added.
The princess holds an important ceremonial role in Thai society — where the royal family sits at the apex, protected from criticism by harsh defamation laws which carry prison sentences of up to 15 years per charge.
Around the capital and across the kingdom, books of well-wishing for her recovery were laid out for Thais.
On Saturday the palace announced that King Vajiralongkorn and his wife Queen Suthida had tested positive for COVID, with both reporting mild symptoms.