The area was sealed off for fear that the 45-year-old two-storey building might collapse following a blaze that burned for an hour.

Police and fire-fighters were alerted at about 7.30am and found the second floor of the building, which was closed earlier this year, fully engulfed.

Pol Lieutenant Chakrapong Pitakkornsakul of Phaholyothin station said the fire was brought under control in about an hour. No one was injured, he said.

Niboon Sangsawan, 39, personnel division chief of Navakit Banmuang Co, said he and three other staff members were working in a storage room when they heard two or three explosions on the second floor at about 7.10am.

A security guard told him there was a fire, so they rushed to the area and encountered a huge blaze and thick smoke.

The fire spread quickly, possibly because of the age of the building, erected in 1972, Niboon said, adding that the second floor was no longer in use.

Jakrapong said initial investigation suggested the cause was an electrical wiring fault in the ceiling of the second floor, which was totally destroyed.

Metropolitan police chief Pol Lt General Sanit Mahatavorn said the building was insured, but he didn’t know for how much.