Internet problems at several companies have been blamed on a fire at Telephone House in Belfast city centre.
Firefighters have tackled a blaze which broke out on the fourth floor of Telephone House on May Street in Belfast city centre at around 7.30am on Wednesday morning.
Police closed May Street, Seymour Street and Gloucester Street to traffic until around 9am.
There were major traffic delays during rush hour while the fire was extinguished.
Fire crews had five appliances and an aerial ladder at the scene.
The roads have since reopened.
Meanwhile several companies have suffered internet problems which they believe to be linked to the fire.
A BT spokeswoman said the fire" disrupted the electricity supply to the building affecting a number of customers in our data centre."
"A fire which occurred in our Telephone House building in Belfast this morning has been extinguished.
"Thankfully there were no injuries and a full investigation as to the cause of the incident is currently underway. The fire disrupted the electricity supply to the building affecting a number of customers in our data centre.
"We’d like to extend our thanks to the Northern Ireland Fire and Rescue service for its prompt response and thank our customers for their patience."
By almost 4pm on Thursday afternoon service was restored to the majority of customers.
An updated statement from BT said: "Following this morning’s incident in Telephone House in Belfast we have now restored service to the majority of affected customers with the remaining to be restored within the coming hour."
On Wednesday morning Translink and the Police Ombudsman office tweeted that their websites were not working.
Belfast City Council also said its website is down.
Tourist attraction Titanic Belfast has also tweeted to say their website is down blaming the fire for the outage.
It replied to a customer saying: "Unfortunately the site is down due to a fire in the BT Telephone House building this morning."
The Grand Opera House website was also down but tweeted to say it was back online just before 2pm.
Council websites have also experienced problems this morning with Antrim and Newtownabbey Borough Council tweeting that their website is down.
PR company JPR tweeted to say their email access has been affected by the fire and advised people to call them if required.