The Thai government says it will be impossible to protect all of the densely populated capital Bangkok from flooding because of a build-up of water to the north.
Prime Minister Yingluck Shinawatra said water would have to be allowed to flow through parts of Bangkok out to sea.
Describing the flooding as a "national crisis", she said officials were doing all they could to solve the problem.
The BBC reports that on Wednesday officials urged residents in seven Bangkok districts to prepare for possible flooding.
Central Bangkok is protected by flood barriers which have been reinforced by troops in recent days.
But the run-off from severe flooding in central parts of the country has built up to the north of the capital, and several northern suburbs are already underwater.
"We cannot block the water forever," Ms Yingluck said. "We need areas that water can be drained through so water can flow out to the sea."
"This problem is very overwhelming. It's a national crisis so I hope to get cooperation from everybody," the prime minister said.
The floods - Thailand's worst in decades - have been triggered by heavy monsoon rain that begin in July.
At least 320 people have been killed over the three-month period, with northern and central areas badly affected in the early stages.
About a third of all provinces are still affected and manufacturing has been hit because a several large industrial estates have been forced to close.
