Devastated Thais held a vigil outside a hospital in Bangkok today where a princess is on a life support machine following a suspected heart attack while running with her dogs.

Heir-to-the-throne Princess Bajrakitiyabha, 44, was taking part in a canine training event when she collapsed and lost consciousness in Nakhon Ratchasima, northeast Thailand, on Wednesday, December 14.

The diplomat and lawyer, who also held a raft of ceremonial military positions, was flown by helicopter to the King Chulalongkorn Memorial Hospital.

Buddhist monks across the country were ordered to pray for her and Prime Minister Prayut Chan-o-cha has flown back early from an overseas trip. Several other senior members of the country's royal family are also returning from abroad, with the palace issuing statements confirming that the princess suffered a medical emergency.

Dozens of monarchy supporters gathered outside and inside the hospital on Friday morning holding portraits of the princess, who was being lined up to replace the aging controversial leader Maha Vajiralongkorn.

Wreaths of flowers were also laid inside the hospital lobby in front of a large shrine made for the royal and palace guards in white military uniforms were scattered around the building and its car parks.

Thai royal watcher Andrew MacGregor Marshall, an author and lecturer, said the prognosis was 'very grim' for the princess who he claims has already died, with an announcement expected 'within days'.

MacGregor claimed the incident has 'huge repercussions' for the Thai monarchy, which punishes critics with prison, and that the institution could be thrown into chaos without a suitable heir.

Crop top king Vajiralongkorn - who reportedly kept a harem of 'sex soldiers' in a luxury German hotel during the pandemic lockdown - has banished his first four male heirs to the United States after a fall-out with their mother and his youngest known male child suffers from severe learning difficulties.

MacGregor said: 'Princess Bajrakitiyabha was initially taken to Pak Chong Nana Hospital where CPR was attempted for hours without success. King Vajiralongkorn rushed to the hospital in a military helicopter.

'Royal sources say the prognosis for the princess is very grim and it's very unlikely she will ever recover. But she could potentially be kept artificially alive via ECMO for weeks or even months while the regime figures out what to do.

'These events have huge repercussions for the Thai monarchy because it was assumed Bajrakitiyabha would be the next monarch or would be regent for her younger half-brother Dipangkorn who has autism and will not be able to reign alone.'