Glasgow City Council officials said that their surveys of the building have shown that there has been substantial movement in the building, meaning a sudden collapse of certain parts of it was ‘likely’.
It marks a change in the rating of the building, from ‘possible’ only last week.
A council statement said: “There is now a requirement to at least partially dismantle sections of the building as a matter of urgency.
“However, the dangerous nature of the site - which includes the Mackintosh building and the O2 ABC - is such that it will take around two days to devise a methodology for taking down the south facade, which is the most seriously affected part of the building.
“The west gable of the Mackintosh building has continued to deteriorate and the east gable has continued to move outwards.
“It is not possible at this time to say exactly when the work will begin, but it will be as soon as possible.”