News: fire crews are battling to save Glasgow School of Art, the masterpiece designed by Scottish architect Charles Rennie Mackintosh, from a fire that is engulfing the building.

The fire is thought to have started in the basement, rising up to consume half of the building before spreading across the roof, with witnesses suggesting that the entire building is now irrevocably damaged.

Students have told the BBC that they believe the fire was started by an exploding projector in the basement, which set fire to expanding foam that was being used for an art installation, just before 12.30 today.

Update: firefighters say they have saved 90 per cent of the structure of the building.

"I think we've lost it. Fires can look worse than they are but the spread of flames right across the top does not augur well at all," Sunday Times architecture critic Hugh Pearman told Dezeen.

"It's an iron framed, stone clad building, so you might expect the shell of it to remain, but of course the whole thing about Mackintosh is all those fabulous painted timber interiors. The most famous part of it is the library, which has only very recently been restored. It's a really, really sad day."

Guardian critic Oliver Wainwright said that today was hand-in day for students at the school completing work for the end of the academic year.

"The GSA is more than a building. It's like seeing your Granda on fire," said Fran Healy, lead singer of Scottish band Travis, who studied at the Glasgow School of Art in the early 1990s.

Twitter users in the Scottish city are posting images showing plumes of black smoke billowing from the building, which was built between 1899 and 1909. One tweeter also reported hearing an "explosion".

"The smoke from the fire at Glasgow School of Art is visible from seven miles away," wrote @idea15webdesign. "This is awful".

"Terrible news: one of our greatest buildings," wrote Paul Goldberger, former New Yorker architecture critic and Vanity Fair contributing editor.

Charles Rennie Mackintosh studied at the Glasgow School of Art before designing the Renfrew Street building, widely referred to as the Mac, in Glasgow's city centre.

Architect Steven Holl recently completed an extension to the school – a matte-glass building that contrasts with the decorative sandstone facade of Macintosh’s masterpiece.

The school is Scotland's only independent art institution and is organised into the three academic departments, the Mackintosh School of Architecture, School of Design and School of Fine Art.

Mackintosh's building primarily houses the Fine Art department, as well as a gallery that hosts exhibitions and the student's annual degree show.

"It is internationally famous – architects around the world pay homage to it," said Pearman. "It is such a significant national monument that money will be found to restore it but it will never be same."

"I remember going through the studios when I went up there this year, and seeing original Mackintosh doors spattered with decades of paint from students, door handles worn by students. It's those marks of time which added to it and made it somehow even better than it would have been when it was brand new."

In 2009 the building won the Royal Institute of British Architect's "Stirling of Stirlings", after a public vote and private judging panel both chose the Mac as the best British building of the past 175 years.

If you are on the scene or have any pictures of the fire, please contact anna@dezeen.com.

Main image is by Andrew McMichael.

Additional reporting by Anna Winston.