Pathumwan district officials sealed off public access to the American University Association (AUA) Language Centre building after a fire at the site on Wednesday.
Chongkolrak Ingturak, director of the district office, said no one would be allowed access to the building on Ratchadamri Road except officials authorised to investigate the fire and whether the site was safe.
If the building's total area is less than one rai it will be declared a "hazardous area" where the public is urged to be cautious even when passing by. If it is more than one rai, it will be declared as a fire area, with the public including AUA employees banned from the area for 45 days. No construction will be permitted during that period.
There were cracks clearly visible on Wednesday, apparently from previous fires, accidents and old age, making it unsafe for investigators even to enter the building.
Wednesday's fire, visible for several kilometres, caused no injuries. It was reported at 12:30pm and at least 10 fire trucks responded. It took an hour to bring the blaze under control.
A preliminary investigation showed the building was in the process of demolition. The fire began in a second-floor meeting room, and flames quickly spread to the third floor.
Narit Chanthapa, a security guard of General Guard Group who was on duty, said he was on the ground floor when he first saw black smoke starting to come out of the building. He rushed in with a fire extinguisher but the smoke was too thick, so he immediately called police.
The cause of the fire was unknown. Mr Narit told police a group of technicians was cutting air conditioning pipes with gas-operated machinery just before the blaze began.
Investigators hope to find the cause of the fire, and also will determine if the demolition team had a permit to dismantle the building.
AUA moved its offices and classes to Chamchuree Square to clear the area for demolition.