Up to 100 firefighters are battling to bring the blaze at a Primark store in the centre of Belfast under control, with reports the building's collapse could be imminent.
A fire engulfed Primark's flagship store in Northern Ireland this morning - with more than a dozen fire crews battling to save the landmark building in Belfast city centre.
It was initially appearing to be confined to the upper floors but flames quickly spread downwards and throughout the entire building.
The BBC is reporting a police officer at the scene has said the building is "at risk of imminent collapse".
Michael Graham, assistant chief fire and rescue officer, said they would continue to deal with the fire into the evening and potentially overnight.
He said he did not know what caused it, nor whether there were fire alarms or sprinklers in place. Nobody was injured.
He expressed "concerns" over the structural integrity of the building, with part of it "prone to collapse" and said a 30-metre cordon had been established for that reason.
"It was a difficult fire and a fire that has continued for approximately six hours," he said.
"It has been a long and arduous six hours for them but we will continue to fight the fire until we extinguish it.
"It is one of the biggest we have experienced in the last two to three years."
He said he was concerned about the front and the two sides of the structure.
Crews are fighting the blaze on several fronts.
The building has been evacuated. Residents and businesses in the city cenre were advised to keep their windows and doors closed as a precautionary measure.
The fire started on the fifth floor of the building, which has been undergoing refurbishment work.
CEO of Primark Paul Marchant said he is deeply shocked and saddened by the fire and moved to reassure staff that have been gravely impacted by the blaze, saying he will ensure they are "fully supported".
Flames reaching at least 50 feet could be seen right across Belfast's skyline.
The historic clock at the top of the building was engulfed in flames before the fire spread from the roof to the floor below.
Parts of the roof then collapsed as thick black smoke and fire ripped through the rest of the flagship store.
The building has survived bombs and fires throughout its long history in the city.
A Primark spokesperson said the shop was safely evacuated this morning 'following a fire on the roof'.
Hundreds of people gathered to watch firefighters tackle the blaze.
Police are also at the scene and have cordoned the area off. They have advised motorists to avoid the Castle Street area.
Diversions are currently in place at Bridge Street and High Street, Castle Street and Queens Street and Royal Avenue and North Street to allow Northern Ireland Fire & Rescue Service to deal with the incident.
Castlecourt and a number of other local businesses have closed due to the fire.
Drivers parked in Castlecourt car park can still access their cars via the car park entrance.
Caitlin Hunter (17) was in the ground floor when the alarm was raised.
“I was just on the ground flooring all of a sudden I could smell smoke. People started walking out quickly and I followed the crowd.
“Before I knew it there was smoke everywhere and flames, even though it seemed to start off very small.
“The mood was very confused in the shop, everyone could smell the smoke but nobody could tell what was going on.”
She added: “It’s a shock and a very scary moment.”
Within minutes the entire building was emptied and the flames tore through the top floor.
Belfast City Councillor Jim Rodgers said he had been told tar was being laid on the roof of the building shortly before the fire started.
A number of loud bangs were heard on the street below, believed to be gas cannisters.
“Inquiries are still at a very early stage but I’ve been told by a number of people that the fire started on the roof, seemingly it was tar being put down on the roof as part of the refurbishment of the building.
“It’s been worked on for so long and within minutes it’s disappeared into an absolute inferno.
“I’ve got to praise the fire and rescue service, there’s a large number of them here and they’ve done a brilliant job in bringing the fire under control.”
Sinn Fein Mayor of Belfast Deirdre Hargey has urged members of the public to keep a safe distance from the site.
“I would also ask the public to avoid the Castle Street and Royal Avenue avoided whilst the incident is ongoing," she said.
“Emergency services at the scene and responding. I would urge the public to be cautious and to assist the emergency services in whatever way possible."
Designed by architect Sir Robert Taylor, the historic Bank Building was constructed in 1785 and opened in 1787. It was later a shop and drapery firm at various points.
The building was extensively damaged by three bombs on April 9 1975, this was followed by a fire which forced then tenants Boots to move out.
It was then refurbished in 1979 and reopened by current owners Primark.