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."
What have eyewitnesses said?
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."
At the scene: Small sections collapsing
By Catherine Morrison, BBC News NI
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.
What is the history of the building?
The Belfast city centre branch of the Primark clothing and homeware chain was undergoing a major refurbishment.
It was being extended along Castle Street in a project costing an estimated £30m.
The original Bank Buildings was designed by Sir Robert Taylor and erected by Waddell Cunningham in 1785.
WH Lynn designed the new Bank Buildings in 1900.
He intended it as a bridge to the 20th Century, with a compromise between the classical style of the upper part of the building and the great expanse of plate glass below.
Three bombs exploded in the Bank Buildings in 1975 - a huge fire broke out shortly afterwards and damaged parts of the building.
In 1979, it was taken over by the Dublin-based group Primark.
Within 18 months, the store was totally refurbished and the exterior restored to its 1903 glory.
What has the reaction been?
Primark CEO Paul Marchant said he was "deeply shocked and saddened" by the fire.
"The safety and wellbeing of our customers, colleagues and construction team is of course the most important consideration and we are truly relieved that all were evacuated safely," he added.
Rajesh Rana, the president of Belfast Chamber of Trade and Commerce, said he was sorry to hear about the fire.
"It is a sad day for Belfast and for one of our city's leading retailers, which was in the midst of completing a £30m expansion project," he said.