A "heartbreaking" blaze at Liverpool's Littlewoods building raged into Monday morning, leaving fire crews battling to save the landmark.
The roof and an upper floor of a wing of the "iconic" building were feared to have been destroyed after flames took hold on Sunday evening.
Merseyside Fire and Rescue Service (MFRS) said crews were confronted with a "significant" blaze that at one point saw an area measuring 200m x 100m alight.
However firefighters were able to prevent the whole site being engulfed and the 1930s Art-Deco structure's integrity was thought to have withstood the blaze.
The former Littlewoods Pools building has lain empty since 2003, although a £50 million scheme to transform the site had recently been announced.
Among the proposals was to build film and television studios at the site, potentially bringing hundreds of jobs to the area.
Mayor of Liverpool Joe Anderson said the fire was "absolutely heartbreaking" and "a real setback".
"Let's hope and pray its salvageable," he tweeted.
MFRS said it was called to the building at 7.52pm and crews were on the scene within four minutes, but the roof of one of the buildings was already well alight.
The fire was declared to be under control at around 1.30am on Monday morning and there had not been any reports of injuries.
Adam Higgins, co-founder of the building's developer Capital & Centric, said the blaze had started in the west wing of the building, which is known for its tower.
Fire crews were able to prevent flames spreading to the east wing, although the roof and an internal floor of the west wing were feared to have collapsed.
"So far all the walls remain intact and obviously they are the bits that are really iconic to people from the city," he told BBC Radio 5Live.
"The building's hugely important to us and to everyone really in Liverpool. We announced some big plans about three months ago which would bring in Twickenham film studios to the city and to this building in particular.
"I'm very hopeful that we still will be able to do that because the walls still seem to be intact and, providing we can keep them up, we can still very much continue with our plans.
"Twickenham are going into the east wing, the building next door, an other companies will be moving into the west wing which has been on fire this evening."
Mr Higgins said investigators had been unable to enter the building due to the collapse and the cause of the blaze was unknown.
MFRS area manager, Paul Murphy, said the size of the building made it "extremely difficult" for crews to deal with the blaze.
"However, crews have worked extremely hard to battle this fire and have prevented it from spreading into the remainder of such an iconic building," he said.
"We are all Merseyside firefighters and this building is as much iconic to us as it is to anyone else, so we are really pleased that we have been able to prevent further spread of fire.
"It has been a challenging incident but we are in a position with the council where we feel the structural integrity of the building may remain intact."
The building on Edge Lane was built in 1938 and was part of the football pools' coupons arm of the Littlewoods empire, built up by self-made tycoon Sir John Moores.
A series of redevelopment plans have fallen through over the years.
Twickenham Studios tweeted: "Very sad news. Hope all are safe and sound tonight. Thinking of you over there."