British travellers have been advised to avoid all but essential travel to Bangkok as tens of thousands of Thais have been ordered to evacuate the flood-threatened city.
The Foreign Office has changed its travel advice in response to the large volume of floodwater expected to hit the capital later this week.
Heavy rainfall across the country has killed more than 350 people and caused flash floods, mudslides and widespread disruption.
Foreign Office Minister Jeremy Browne said: "Our thoughts continue to be with the people of Thailand at this difficult time and in the face of unprecedented flooding.
"However, as prime minister Yingluck Shinawatra has now said that waters will reach the capital, we have updated our travel advice to reflect the worsening situation."
Bangkok governor Sukhumbhand Paribatra told residents of Don Muang and Bang Phlat districts, which are already partly submerged, to leave for their own safety.
He said: "This is the first time I am using the term 'evacuation', the first time I'm really asking you to leave."
Satellite images of Bangkok showed a city almost completely surrounded by water.
The Government of Thailand has announced a public holiday from October 27-31, when the floods are expected to hit the capital.
Bottled water and tinned foods are already selling out as residents prepare for the flooding, which is expected to disrupt transport and may affect electricity and water supplies. Tourist attractions are likely to be closed.
Travellers have also been advised to stay clear of the 26 provinces affected by flooding. British nationals already in Thailand are urged to be cautious.
September and October are the peak months of the monsoon season in Thailand.