Amid concerns the A-listed structure could collapse “with no warning” as a result of extensive damage, Glasgow City Council leader Susan Aitken said investigations by building control experts showed it had “moved quite significantly”.
The confirmation comes as David Mundell, Secretary of State for Scotland, became the latest high-profile figure to call for the landmark to be rebuilt, although he admitted its damaged state was “utterly shocking to behold”.
After three days of work by the council’s building control team, Ms Aitken said there were fears the remains of the building, known as the Mack, could fall down at any moment.
She said: “Our building control officers are saying part of the building has moved quite significantly, about six inches.
“The elevation has shifted. There’s a danger that part of the building may collapse.
“If that were to happen, it could happen with no warning. Therefore it’s very, very important that no-one enters the cordon unless they are an accredited expert.”
Mr Mundell, who visited the site yesterday alongside Ms Aitken and Professor Tom Inns, the art school’s director, said he echoed their belief the stricken Mack could be saved.
He said: “I share their conviction that the Mack will one day rise again in all its glory.
“It is still early stages in properly assessing the damage, but we all hope the building can be saved and the UK government stands ready to help.”
Mr Mundell said the sight of the fire-damaged building was one of “devastation”, adding: “Having seen the splendour of the painstakingly refurbished building just two weeks ago, it is an absolutely heartbreaking sight.”
The local authority’s building control department is working with the GSA, expert structural engineers, David Narro Associates and Historic Environment Scotland.
That work will predate any further investigations about what has been lost in the fire.
Artefacts feared to have been lost to the flames include statues that inspired generations of artists. The plaster casts of famous works, including Michelangelo’s Madonna and Child, survived the 2014 fire that caused widespread damage to the building, but the recent blaze was more severe.
While some of the plaster figures, friezes and busts had been moved into storage in the nearby Reid Building after the first fire, others remained in situ, in part because they were considered too fragile to be removed.
A spokeswoman for the GSA confirmed some of the casts were in the Mackintosh building, but said it was too early to determine what has been lost in the fire.
Meanwhile, looters have broken into a bar that has been shut since the incident.
Police Scotland received a report of a break-in at Campus on Sauchiehall Street around 3:20am on Thursday.
The bar is inside the cordon that was put up to seal off the busy city centre area. A number of other businesses remain closed.