While much of the building appears to be intact, initial reports suggested that the celebrated library, an internationally recognised beacon of Art Nouveau decoration and furnishing, and the studio above it, both housed in the west wing, were entirely lost in the blaze on May 23, along with its contents and numerous artworks.
The Hen Run, a glass-fronted gallery on the top floor providing inspirational views for artists and designers across the city, which housed many of the studios for women, was also destroyed.
The Mackintosh Library was home to a number of the larger, oversized books from the GSA's Special Collections, along with all their pre-1985 journals. It also housed publications on and by the GSA's staff and students, both past and present, along with graduating students' degree show catalogues and rare and archival items, such as periodicals dating back to the early 19th century and publications about the architect/designer Charles Rennie Mackintosh himself.
An early estimate of the loss puts its value at around £50m.
Historic Scotland has sent in specialist conservation staff to work with GSA archivists to see what can be salvaged and conserved.
Scotland's First Minister, Alex Salmond, has announced that as a first step the government will pledge up to £5m in matched funding for the GSA's restoration fund.
As the GSA authorities set their emergency recovery programme in motion, they released a statement saying: "The impact of the Mackintosh Building fire on students, staff, the city and the wider creative community cannot be underestimated and this is a complex and rapidly changing situation."
Muriel Gray, the broadcaster and former GSA student who chairs the governing body, praised the fire service for protecting the vast majority of the building and the archives, which had been stored elsewhere.
"As for the library, Mackintosh was not famous for working in precious materials. It was his vision that was precious and we are confident that we can recreate what was lost as faithfully as possible," she said.
Designed by Mackintosh and opened in 1909, the A-listed building is one of Scotland's best-loved landmarks and widely acknowledged as his masterpiece.
Those wishing to contribute to the initial £1m restoration appeal fund may do so online via www.theBigGive.org.uk