More than 120 firefighters were called to the historic college last night, Friday 15 June 2018, to tackle a blaze that is understood to have spread throughout the entire building.
Peter Heath, deputy assistant chief officer of the Scottish Fire and Rescue Service, told the BBC that the fire "had seriously and significantly engulfed a large part of the building".
"The fire was extensive, the conditions were very arduous and it was clear that this was going to be a very serious and ongoing incident," he said. "The building has been extensively damaged."
The fire had spread from the ground floor all the way up to the roof, and had also reached nearby buildings, the Campus nightclub and the O2 ABC music venue.
Glasgow School of Art revealed the news via its Twitter feed yesterday: "There is a fire at the Mackintosh Building. The fire brigade are currently on scene. We will report back with news updates as soon as possible."
— Banpo & Monkey (@Banpo_Monkey) June 15, 2018
Videos and photographs posted on Twitter showed large flames visible above the roof.
The new blaze comes as a major restoration led by architects Page\Park was nearing completion. The £35 million project, due to complete next year, involved recreating the library, the most celebrated room at the school, which was destroyed in the previous fire on 23 May 2014.
According to Heath, all the restoration work is likely to have been destroyed in the latest fire.
Glasgow School of Art was completed in 1909 and is the best-known building designed by Mackintosh, Scotland's most famous architect.
Last week was the 150th anniversary of Mackintosh's birth. Dezeen published a special series to mark the occasion.
Image is by Calum Tomeny.