Six fire appliances and an aerial ladder at the blaze at BT's Telephone House on May Street in Belfast city centre
A fire at a BT building in Belfast affected its data centre and "had the potential to disrupt access to certain websites", the company has said.
The statement followed a day of disruption for several high-profile organisations in Northern Ireland, who reported problems with their websites.
The fire on the fourth floor of BT's Telephone House, on May Street, was reported just before 06:00 BST.
It was extinguished by fire crews who are treating the blaze as "accidental".
BT said its data centre services have now been fully restored.
Standstill
Organisations that reported website problems earlier on Wednesday included Translink, the Police Ombudsman's Office, external, Belfast City Council, Northern Ireland Electricity, and Budget Energy in Londonderry.
Anne McEvoy, Budget Energy's operations manager, told the BBC's Evening Extra programme that her firm's website was down all day, meaning its own online services were "at a standstill".
Her firm had to deploy extra customer services staff to deal with clients who could not buy electricity online.
Fire officers tackled the blaze using an aerial appliance
She said the website was not restored until shortly before close of business at 17:00 BST.
A BT spokeswoman told the BBC on Wednesday evening that the company was not able to say how many internet customers had been affected by website problems.
In a statement, she said: "The fire disrupted the electricity supply to the building, affecting a number of customers in our data centre.
Smoke-logged
"This had the potential to disrupt access to certain websites during the day.
"Data centre services have now been restored to all those affected and BT continues to monitor the situation."
Eight firefighters wearing breathing apparatus entered the smoke-logged building on Wednesday morning to extinguish the blaze.
Six fire appliances and an aerial ladder attended the scene.
Kevin O'Neill, an area commander for the Northern Ireland Fire and Rescue Service, said it was a "small fire", but because of the materials involved two floors of the building sustained heavy smoke damage.
Buildings in the area were evacuated during the operation, and there were rush-hour traffic delays in the city centre.
May Street, Seymour Street and Gloucester Street were closed for a time, but reopened later on Wednesday morning.
