There are "grave concerns" that the Primark building in Belfast city centre could collapse, the fire service has told the BBC.
A fire broke out in the listed five-storey Bank Buildings at about 11:00 BST on Tuesday.
Firefighters remain at the scene and the roof and some floors have collapsed.
There are no reports of any injuries and it is not yet clear how the fire started.
The fire service said the blaze started on the fifth floor or the roof.
The flames died down on Tuesday evening but firefighters are expected to remain at the scene into Wednesday morning.
Firefighters have pieced together 1km (3,280ft) of hose to bring water from the River Lagan to the building.
"We certainly can't guarantee the structural integrity of the building at this time," said Assistant Chief Fire Officer Michael Graham.
"Our expectation is that the internal floors will collapse where the fire has been burning at its greatest intensity.
"We actually hope that might help the overall structure."
A police officer at the scene told BBC News NI earlier on Tuesday that structural engineers had assessed the building and said it was "at risk of imminent collapse".
Police are advising people to avoid Castle Street and Royal Avenue.
Royal Avenue has been cordoned off and pieces of debris have fallen from the side of the historic building.
Clothing giant Primark confirmed that the building was safely evacuated.
It is believed that more than 350 people work in the store.
Mr Graham said it had been "an intense six hours" for firefighters.
"The priority for us was to stop any fire spreading to any other building and keep our firefighters and the community safe," he added.
"We feel we've achieved all of those things."
Alice Murdoch told BBC News NI that she was inside the building just before it was evacuated.
"I just popped inside and you could smell the smoke already," she said.
"Staff were sort of looking at each other and then the alarm started going off and they said: 'Get out as quickly as you can.'
Ms Murdoch said it was "quite scary" but that the evacuation was well organised.
Caitlin Hunter said there was a sense of "controlled panic" as people inside the store tried to get out.
"I was inside on the bottom floor just having a nosey around at the make-up section and all of a sudden I could smell a weird smell of smoke," she said.
"Nothing had been said at this point what about was happening but people [had] their gut instinct, knowing something wasn't right.
"It was a race for people to come down the lifts and stairs... we were all rushing out."
A large crowd of several hundred people has gathered at the police cordon on High Street as firefighters continue to tackle the fire.
They are watching one of the biggest fires in Belfast city centre in peacetime.
There are gasps from the crowd as small internal sections of the building collapse every few minutes.
Flames are now contained to a small section of the third and fourth floors.
But the fire service has said it will continue with this operation for the next three to four hours.
"The Bank Buildings was a five-storey, Grade B1 listed red Dumfries sandstone building located at 1-27 Castle Street in Belfast, Northern Ireland. The building was owned by Primark, and was used as their flagship store in Northern Ireland from 1979 until it was destroyed by fire on 28 August 2018.
As a part of renovations between 2016 and 2018, 29-43 Castle Street (formerly Commonwealth House) was demolished and a new building built in its place which was set to be merged with the original Bank Buildings. The new wing of the building appeared relatively undamaged following the fire of August 2018. The fire damaged Bank Buildings is currently undergoing a complete restoration and repair of the building.
On 28 August 2018, a fire broke out at roughly 11:00 BST on the fifth floor or roof of the building, during business hours. Shoppers and staff were evacuated from the ground and first floors, and 11 fire appliances attended. The area around was cordoned off for public safety, and due to falling debris. The roof collapsed and the clock face was burnt out with the hands staying still at about 11:05. The fire was still spreading and had covered all floors of the building by 15:00 with more appliances called in to assist. The building suffered extensive damage to all levels. The new wing of the building appeared relatively undamaged following the fire, due to the fact there was a gap between the two as the merge had not yet been complete. On 9 October 2018 it was reported that Bank Buildings is currently awaiting approval from Belfast City Council for an application of planning permission for a complete restoration undertaken directly by Primark of the fire damaged Belfast Bank Buildings. This work was green-lit on 26 October 2018, with the initial stages aimed at reducing the safety cordon due to commence ''immediately''. It is understood that this will involve the removal of the top two storeys of the building, deemed to be the most unstable, which will then allow a façade retention scheme to be put in place. The stonework recovered during the removal of the top two storeys will be numbered and labelled, to be reused later in reconstruction. The iconic clockface will also be removed for restoration and eventual reinstatement.
England based demolition firm Keltbray have been appointed to carry out the emergency safety works, stone salvage, façade retention and structural demolition works.
On 3 December 2018 Keltbray were successful in delivering the first milestone of the project, the area around Bank Buildings was reopened to pedestrians, with concrete-filled shipping container tunnels being used to protect the public from any potential collapse.
Phase 1 of the salvage works (removal of levels 4 and 5 façade) was completed by Keltbray ahead of schedule.
Keltbray have now commenced the installation of the Façade Retention System that will stabilise the building and allow the internal demolition to commence in mid 2019."