AS pockets of fire continued to burn last night in the smouldering ruins of the Glasgow School of Art, despite the constant streams of water cascading down on them, it became clear that the building was beyond salvation.

No one will say officially that Charles Rennie Mackintosh's Grade A-listed masterpiece is doomed, but firefighters at the scene described the remaining structure as a "tottering shell" and "beyond redemption".

One, who didn't want to be named and who had fought the previous fire in 2014, said: "This is so much worse. I've been involved in fighting many, many fires so I'm a pretty fair judge. The structure is clearly unsafe and I don't see any way it can be saved, except taking it all down and rebuilding it stone by stone, which would be astronomically expensive."

Another told the Sunday Herald: "We haven't been able to get inside the building yet, for obvious reasons. All of the floors have gone it seems and the roof is in. This fire has gone throughout the building which didn't happen with the previous one. The severity of it, the way it took off was much greater. It was as if it had been firebombed."

Asked whether he knew if the building had a sprinkler system or if that could have made a difference, he said: "It's impossible to say, perhaps in the very early stages, but it flared so quickly and so intensely I don't think anything would have worked."

The Scottish Fire and Rescue Service incident commander, David Young, described the damage as "considerable", but refused to answer whether the building had sprinklers and smoke alarms. The Art School also failed to confirm or deny despite repeated requests.

A spokeswoman, in a formal statement, said only: "The GSA and all of its buildings will remain closed for the next week, and we will provide updates as and when information is available."

The blaze which tore through the Art School's Mackintosh Building in 2014 – "the Mack" –began when a projector ignited gases from expanding foam used in a student project, a fire investigation confirmed.

The flammable gases from a foam canister caught fire when they came into contact with the electrical equipment as work was being carried out on a nearby art installation.

Young would not speculate on the cause of this fire, saying only that fire investigators were on the scene and would begin their detailed work when it was safe to do so.

Alan Dunlop, the school’s professor of architecture, said: “I can’t see any restoration possible for the building itself. It looks totally destroyed.”

Scotland's First Minister Nicola Sturgeon, who was given a first-hand tour of the devastation yesterday afternoon, said the blaze which ripped through the historic building had left it like a shell, adding that it was "heartbreaking". It was, she said, "much, much worse" than the first fire four years ago.

Asked about the future of the building, she said: "I've spoken to the principal of the Art School already and the Scottish Government stands ready to do anything we reasonably can to help ensure that the building has a future.

"It's too early to say what that might entail or what that might look like. We don't know yet what the structural condition of the building is. It's simply too early to give definitive answers but I'm determined as we were after the fire four years ago that the Scottish Government will do everything it possibly can."

The building was less than a year away from reopening after a £35 million renovation programme which brought in donations from across the world, including from Hollywood star Brad Pitt.

Margaret Archbold, 48, a Glasgow artist who studied fine art at the school and graduated in 1994, said: "It shouldn't have happened again. It was graduation day yesterday [Friday] for this year's students. I just feel really sorry for the fire brigade because they worked so hard to save it the last time.

"I came down to say goodbye actually, I thought there was nothing left, and it was quite an important time in my life."

Stuart Robertson, director of the Charles Rennie Mackintosh Society, said the fire was "terrible" and a "massive body blow", and that it would be "sending shockwaves around the world".

He said: "I've only seen glimpses, and what the firemen are saying, reading between the lines it looks bad. The last time they stopped it going into the east wing, this time it looks as though it's gone from the east wing all the way through."

The fire also spread to the adjoining O2 ABC venue, where the roof collapsed inside the building, leaving its future uncertain. Firefighters on turntable ladders were still hosing the smouldering building as darkness fell last night.

Scottish Secretary David Mundell said: "I am devastated to hear the news of fire at Glasgow School of Art. It is only a fortnight since I was there to see the progress on the restoration, and my heart goes out to all those who had worked so hard on that."