Thais have grown increasingly on edge over the health of revered King Bhumibol Adulyadej, who has been hospitalised for much of the past year with a series of illnesses.

The palace has increased the number of health updates on the ailing 88-year-old monarch, and on the weekend issued a statement saying his condition was unstable.

The palace issued a further statement last night, saying his condition had not improved.

In a day of drama yesterday, Thai Premier Prayut Chan-o-cha abruptly cut short a provincial trip to return to Bangkok, fuelling speculation.

At the same time, all of the King's children, including heir apparent Crown Prince Maha Vajiralongkorn, turned up at Siriraj Hospital in Bangkok, said reports by Khaosod newspaper.

The Crown Prince also cancelled an appearance at a university graduation ceremony scheduled for later this month.

Many Thais took to Facebook or Twitter to express their concern, with a hashtag of "long live the King" in Thai.

While government spokesman Sansern Kaewkamnerd later told reporters that the Prime Minister had returned to Bangkok to prepare for an audience with the Crown Prince "for a routine presentation on the government's work in progress", it did little to quell rumours, particularly given the increasingly serious nature of the palace updates.

This was especially so after the statement issued by the Royal Household Bureau last night, which said the condition of the world's longest-reigning monarch had "overall not yet stabilised".

Anxious Thais have camped out in the courtyard of Siriraj Hospital, awaiting news of the King's condition and praying for his recovery. He has spent much of the past year at the hospital. Thais have been wearing pink, considered an auspicious colour for the monarch, in the hope that he will recover after a long period of illness.

The latest update said the King's blood pressure was found to be lower on Tuesday, and his pulse faster.

"The result of the blood test showed that there was an infection, and his liver was not in a regular condition," it said. Hence doctors have given him antibiotics, put him on a ventilator, and also on continuous renal replacement therapy (CRRT).

CRRT is usually given to critically ill patients.

King Bhumibol is seen as a demigod and father figure in Asean's second-largest economy and his successor has big shoes to fill. But a powerful palace machinery also keeps tight control on news of the royal family, and critical discussion about the monarchy is curbed by a lese majeste law that imposes up to 15 years' jail for insulting or defaming the king, queen, heir apparent or regent.

Over the past few months, King Bhumibol has been treated for a range of ailments, including lung infection and renal failure, as well as hydrocephalus, a condition commonly referred to as "water on the brain".

Thai stocks, which have taken a battering since Monday, slid further into negative territory. Thailand's benchmark SET Index, which shed 3.15 per cent on Monday and 1.02 per cent on Tuesday, plunged by more than 6 per cent after lunchtime yesterday but recovered to a 2.5 per cent drop at the close of trading.

In contrast to the longevity of King Bhumibol's reign, Thailand has seen many short-lived governments and regular military coups, the most recent in 2014 which overthrew an administration controlled by the popularly elected Puea Thai party.

Uncertainty over the impending transition has helped fuel the political conflict that has embroiled Thailand in the past decade, though the current military rule is expected to keep the lid on tensions in the near term.