Some shops in the Thai capital began imposing emergency rationing on Wednesday as anxious residents stockpiled food and other necessities, with Bangkok under threat of inundation for weeks to come.
As panic mounts, shoppers at a central Bangkok hypermarket run by Big C Supercenter Pcl were being restricted to one packet of rice and one tray of eggs. Toilet paper was also being rationed.
Bottled water had run out at the hypermarket, except for expensive brands, compounding fears that residents would have access to adequate supplies during the crisis.
In some areas, people are already complaining about a deterioration in the quality of normally drinkable tap water.
The Metropolitan Waterworks Authority said floodwater had got into raw canal water used for its supply. Chemicals were being used to purify it.
Shares in Thai beverage firm Haad Thip Pcl surged more than 6 percent on Wednesday on expectations the company would benefit from the huge demand for drinking water.
The floods in the north, northeast and centre of Thailand have killed at least 373 people since mid-July and disrupted the lives of nearly 2.5 million, with more than 113,000 in shelters and 720,000 people seeking medical attention.
Authorities have called a holiday from Thursday until Monday to allow people to get out of Bangkok as a record high tide approaches in the Gulf of Thailand, although financial markets will remain open.
With the high tide approaching, Seri Supharatid, director of Rangsit University's Centre on Climate Change and Disaster, said the city's fate rested with dikes along the river.
In the worst-case scenario, if all the dikes break, all parts of Bangkok would be more or less flooded, Seri said.
The rising tide could complicate efforts to drive water from the swelling Chao Phraya river out to the sea, putting central Bangkok under more pressure. Heavy rain could also deepen the crisis and thunderstorms were forecast for Wednesday.
Authorities are scrambling to pump water around the east and west of Bangkok into the sea.
Prime Minister Yingluck Shinawatra warned parts of the city could face inundation for up to a month.
After assessing the situation, we expect floodwater to remain in Bangkok for around two weeks to one month before going into the sea, Yingluck told reporters on Wednesday.
The situation shouldn't be as serious as in other provinces. We shouldn't face water as high as two or three metres staying for two or three months as we've seen in other provinces.
She had said in a televised address late on Tuesday that central Bangkok could be swamped by up to 1.5 metres (nearly five feet) of water in some places if barriers broke.
The authorities would guard important places such as the royal palace and power stations, she said.
TV footage on Wednesday showed people wading knee-high alongside the river by the Grand Palace, one of the main tourist attractions in Bangkok.
Tourism Minister Chumphol Silpa-archa said tourist arrivals could be 500,000 to 1 million below the government's target of 19 million this year because of the floods.
Don Muang airport, used by budget airlines for domestic flights and by private planes, had to close on Tuesday due to flooding that was making access to the area difficult.
By Wednesday, floodwater had reached the runway, Thai media said. Don Muang plans to reopen on November 1.
The government has its flood crisis centre at the airport and has decided to stay put for now.
Shares in Thai Airways International Pcl slid 2 percent and Airports of Thailand Pcl dropped 1.3 percent on Wednesday due to the Don Muang closure and the possible reduction of Thai Air flights from the main Suvarnabhumi airport, which remained open.
As water levels climbed in Bangkok, some people were being evacuated for a second time, with 4,000 sheltering in Don Muang moving to the province of Chon Buri. Evacuees at a university in Pathum Thani province also had to move on as floodwater engulfed the campus.
Seven industrial estates in Ayutthaya, Nonthaburi and Pathum Thani provinces bordering Bangkok have been closed, causing billions of dollars of damage, disrupting international supply chains for industry and putting about 650,000 people temporarily out of work.
The cabinet has approved a $325 billion baht ($10.6 billion) package to help firms and individuals recover.
(Writing by Alan Raybould and Martin Petty)