David Mullane, a former director of the Charles Rennie Mackintosh Society, who chairs the Friends Of The GSA organisation, believes a rebuilt version of library could be an "embarrassment" and a prime example of "Mockintosh".
The Art School is to hold a symposium on the question of how to replace the library - which was gutted in the fire on May 23, although the archives were saved - but Mr Mullane believes Mackintosh's original cannot be "effectively replicated".
He will argue his case, speaking in a personal capacity, at a debate to be held by the Charles Rennie Mackintosh Society on the future of the library on September 25. It will be held at the Mackintosh-designed Queen's Cross Church in Glasgow.
The Art School intends to "restore and rebuild the Mackintosh Building, including the iconic library, and to rebuild well", a spokeswoman said last night.
However, Mr Mullane said: "I believe works of art, if damaged, can be restored.
"The library, however, has been completely destroyed and the library is a work of art.
"It is Glasgow School Of Art's intention the library will be rebuilt, based upon drawings and photographs.
"It is my contention it may well be an embarrassment, which will be referred to as Mockintosh."
He believes other artists and Mackintosh fans feel the same way about the design classic, although he realises his view is not shared by all.
Mr Mullane believes that instead of following the plans of Mackintosh and rebuilding the library as it was, the GSA should commission a modern architect to create a new space for students to work and converse.
"When I speak to students, they say they would like a high functioning space, somewhat like a common room - in Mackintosh's designs there were little booths, but we need more than that these days," he said.
"I would love to see a tender put out to attract one of the creative architects working in the world today.
"That seems preferable, to be proud of that - rather than constantly apologising beause the library burnt down and it had to be rebuilt."
At the debate, entitled Reinstate Or Reinvent: A Debate On The Future Of The Gsa Mackintosh Library, the opposing view will be presented by Michael Davis, a historian and architectural conservationist.
A spokeswoman for Art School said: "It is the intention of the Glasgow School Of Art to restore and rebuild the Mackintosh Building, including the iconic library, and to rebuild well.
"We are in the process of appointing an architectural practice to lead the restoration and rebuild of the Mackintosh Building, and will be holding two international symposia to inform the restoration process."
At the forthcoming Venice Biennale Of Architecture, the first of two symposiums will be held to discuss the restoration of the Mackintosh Building, with the second being held next spring in Glasgow.
Professor Christopher Platt, head of the Mackintosh School Of Architecture at GSA, said: "What should the plans be for bringing the Mackintosh Building into full use once more and how should we approach the particular issue of the Macintosh Library?
"These are highly complex questions and by necessity any discussion must involve contributions from many different people and organisations from across the world.
"We will therefore host two symposia, the first in Venice this autumn and the second in Glasgow next spring, to explore the future of a space beloved by so many."
He added: "We have decided to focus on the topic of the Mackintosh Library specifically as this is the most delicate and culturally and intellectually significant aspect of the entire post-fire work."