By Mark Gregory
Business reporter, BBC World Service
The head of Thailand's stock exchange advised investors to trade cautiously
Share prices have fallen sharply in Thailand amid concern about the health of King Bhumibol Adulyadej, who has been in hospital for nearly four weeks.
In the latest trading session, the Stock Exchange of Thailand fell 8%, its biggest one-day drop since the height of the global financial crisis in 2008.
The king was admitted to hospital with fever, fatigue and lack of appetite.
The royal household says the 81-year-old monarch has been asked by doctors to remain there for further treatment.
Foreign investors flee
Shares plummeted in Bangkok on a day that saw substantial gains in other Asian financial centres.
The drop was driven by worries about the health of the much-revered King Bhumibol, who was admitted to hospital on 19 September.
His health is a highly sensitive topic in Thailand and the longer he remains in hospital, the more jittery markets are likely to become.
The president of the SET has advised investors to trade cautiously and not to panic during this period.
The Bureau of the Royal Household said in a statement on Thursday that the king's general condition was good, but that he needed more time to recuperate.
"The Committee of Royal Physicians reported that his majesty the king is able to partake more of his meals. Multi-vitamins and rehabilitation therapies are still being given," it said.
Much of the selling pressure has come from foreign investors withdrawing their money from Thailand. Thailand's currency, the baht, has fallen to a two-week low against the dollar.
King Bhumibol is seen as unifying figure in a country that has had repeated coups and coup attempts and 27 different prime ministers during his 60-year reign.
He is the world's longest serving monarch.