Floodwater encircled two industrial estates in the east of Bangkok on Monday and closed in on the centre of the capital, disrupting bus services, although mass transit train systems were still running and commercial districts remained dry.
Starting in the north and northeast of the country in late July, the water has flowed slowly south, overwhelming industrial provinces and rice areas in the centre before moving into Bangkok over the past three weeks.
It is now only about 7 km (4.5 miles) from the Silom business and entertainment district at the heart of Bangkok.
Somkid Tanwatanakul, deputy governor of the Industrial Estate Authority of Thailand (IEAT), told Reuters floodwater had reached the vicinity of the Lat Krabang Industrial Estate in the east of the city and was now about 1.5 metres high.
We have strengthened dikes around the estate to 2.6 metres high. My worry is if this much water continues to hold for a long time with nowhere to go, the estate might not make it.
The estate has 254 factories with almost 50,000 workers. Consumer goods giant Unilever Pcl, Johnson & Johnson, Isuzu Motors and Honda Motor Co all have plants there.
It is about 10 km (6 miles) north of Bangkok's main Suvarnabhumi airport, which is functioning normally inside a reinforced dike at least 3 metres high.
Flag carrier Thai Airways International Pcl said on Monday it had moved its ticket office from its headquarters in northern Bangkok to Suvarnabhumi and two other locations because access had become difficult.
Bangkok's old Don Muang airport, still used for internal flights, has been closed since October 25.
Honda, which was only just recovering from Japan's earthquake in March, has suffered more than most. It withdrew its earnings forecasts on October 31 after it was forced to close its car assembly plant in Ayutthaya.
It also closed its Thai Honda Manufacturing plant last month at Lat Krabang, which produces motorcycles and power products, and decided to extend that suspension on Friday.
The problem with Thai Honda has been the difficulty for staff to get to work and parts supply disruptions, said Waraporn Pongpaiboon, an official at its regional headquarters in Bangkok, adding water had not got into the plant.
Bang Chan, another estate north of Suvarnabhumi airport, is also threatened. With 93 factories and about 13,000 workers, it is home to food firms such as Nestle SA and President Bakery Pcl, which makes buns for McDonald's.
Our factory is still dry but outside, it isn't, said President Bakery marketing manager Yaowaret Kanjanachotkamol. We have started to see water in some parts of the estate.
Filthy water has flooded into the streets of the estate through a canal and drains but factory compounds are dry.
We started to cover the factory compounds with iron flood barriers after we heard about the flooding in Ayutthaya, said Charo Kritchankran, a factory manager for food and electrical equipment producer Saha-Union Pcl.
Floodwater has started to recede in Ayutthaya province -- the site of World Heritage-listed temples and an ancient capital as well as huge industrial estates -- but at the same time the danger is rising in Bangkok.
The government's flood crisis centre said residents of 11 of Bangkok's 50 districts had been told to leave as of Sunday and in another seven a partial evacuation had been called.
But it said its use of so-called Big Bags -- huge sandbags weighing 2.5 tonnes -- to build a protective wall 18 km long across the north of the city appeared to have been successful in reducing flows into the inner city along the first 6 km constructed.
The government says 506 people have been killed and 25 of the country's 77 provinces are currently affected.
The Chatuchak district in northern Bangkok was among the latest to be issued with an evacuation order as floodwater moved in over the weekend, although its huge market, popular with tourists and locals alike, remained open.
The overhead Skytrain, whose northern terminus is by the market, is running normally, as is the underground MRT system, which goes through the area.
But many poorer residents rely on buses to get around and they were having more trouble.
Saitarn Siriatcharanon, 56, told Reuters she had been struggling for six hours to reach her son in the flooded area, whereas normally the same journey would take 30 minutes.
Bangkok Mass Transportation announced the suspension of bus services in the flooded areas, with military trucks offering an alternative in places.
I think it will take at least a month for the situation to get better, said student Tanida Aupornrungrat, a view shared by the authorities, who are having to import pumps to help.
Floodwater is heading towards Victory Monument, where many buses leave Bangkok for other towns. Rama II Road, a major highway to the rubber-producing south of the country, which has not been affected by the floods so far, is also threatened.
(Additional reporting by Panarat Thepgumpanat, Wilawan Pongpitak and Jutarat Skulpichetrat; Editing by Sugita Katyal)