Heavy traffic has been reported in Wattala town along the Negombo - Colombo Main Road.
Police said the lane from Colombo to Negombo has been closed due to a three storey textile shop in Wattala was damaged by the fire.
Vehicles traveling from Colombo can use the alternative route along the Awariwatta Road.
Five service vehicles have been deployed to extinguish the fire, police said.
The cause of the fire is not yet known.