Professor Tom Inns, director of the GSA, said the cash was a "huge boost" for the £20m Art School appeal.
The money will help repair and restore the A-listed city landmark.
As reported in late editions of yesterday's Evening Times, Chief Treasury Secretary Danny Alexander announced the cash pledge.
Professor Inns said: "The UK Government's funding announcement is a huge boost to the GSA Development Trust's Mackintosh Appeal.
"We are determined to restore the building to its former glory, as the inspirational home for the GSA's creative talent and for the delight of visitors.
"The UK government's support will help us enormously in our efforts to rebuild."
Details of the financial support, comes after a pledge of up to £5m from the Scottish Government.
Mr Alexander, said: "The Glasgow School of Art is one of Glasgow's great institutions.
"The response to the fire from people across the world has been phenomenal.
"I am delighted to be able to announce that the UK Government will be providing £5m to the Mackintosh Appeal to help the GSA recover and then continue to thrive.
"We will work with The Glasgow School of Art to ensure that the funding is provided at the most appropriate time to meet their needs."
Firefighters managed to save 90% of the world famous building and 70% of its contents, but it is estimated it could cost up to £35m to restore.
The library was destroyed, but the Mackintosh Museum and the archives survived.
Hollywood star Brad Pitt and Scots actor and former GSA student Peter Capaldi are among those who have given their backing to a £20m fundraising campaign to restore the structure.
Colin Grassie, chairman of The GSA Development Trust, said: "We are delighted with this significant boost from the UK Government, reflecting as it does the recognition of the reach and importance of The Glasgow School of Art.
"On behalf of the trustees, I would like to express our sincere gratitude."