A lengthy probe by the Scottish Fire and Rescue Service (SFRS) into the “catastrophic” incident in 2018 failed to identify how it started.
The investigation found wilful fire-raising, electrical failure and accidental ignition, including by a discarded cigarette, could not be ruled out as possible causes.
However, the inquiry report “strongly suggests” that lessons are learned from the blaze, stating there was “little doubt” that buildings were at greater risk of damage once construction work was underway.
The managers of major construction projects are urged to “fully consider the risk of fire” in future in the official report, which said they should appoint “competent persons with the appropriate skills, knowledge, qualifications and experience to ensure the existing regime of health and safety legislation and regulations are appropriately applied”.
The report also calls for fire safety training for all construction site staff, and “robust checking and monitoring” of works to ensure risks are identified at an early stage.
The investigation report stated the Mackintosh Building was in a vulnerable condition and at higher risk of fire due to the restoration work following the 2014 fire.
It said the ducts that were part of the ventilation system probably helped the fire spread.
Glasgow School of Art chiefs, who announced in October they would be prioritising a “faithful reinstatement” of the Mackintosh Building, pledged to carry out a “lessons-learned exercise” on all aspects of the restoration project.
However, Professor Alan Dunlop, one of Scotland’s leading architects and an outspoken critic of the art school, has said it was now clear the building could not be replicated.
He said: “The investigation has clearly been a challenge and the outcome, though disappointing, is a highly detailed and forensic analysis.
"Unfortunately, no conclusion as to the ‘possible origin’ or cause has been made. This leaves us with many questions and with a less secure foundation to move forward.
"However, the report makes a series of recommendations concerning fire awareness and safety in construction.
"The report confirms that the fire was a ‘significant and catastrophic’ incident and that the building's original ducting ‘served to intensify the fire, promoting uncontrolled fire growth and rapid development’.
"Planning for 21st-century building and fire regulations did not form part of Mackintosh's creative agenda. The sweeping entrance staircase through the foyer then up onto the open first floor gallery; the double height studios with open access corridors; open staircases either end; double height landings and innovative plenum system could not be replicated without much compromise.
"It must remain a working building that meets the needs of a contemporary art school and combines the new with all of the original structure that can be saved and Mackintosh's unique legacy is respected.”
Christina Grainger, president of the Royal Incorporation of Architects in Scotland, said: "The Mackintosh Building is a true Scottish icon and one of the world’s architectural treasures.
"We’d like to see the Mackintosh Building project create opportunities for today’s emerging Scottish architects and craftspeople. Hopefully, the report will allow those efforts to move forward, and we’re ready to assist Glasgow School of Art in finding the right solution to this hugely-important building."
Hundreds of tonnes of rubble were searched for months in the aftermath of the fire on June 15, 2018 at the site where the Charles Rennie Mackintosh-designed building was being restored following a previous blaze in May 2014.
Ross Haggart, deputy chief officer at the SFRS, said: “Our investigation has been a deeply complex and protracted process.
“Due to the nature of the ongoing restoration and construction works combined with other factors such as the air supply into the building, the fire was unfortunately able to take hold, spread and ultimately cause catastrophic damage.
“This presented a number of challenges to our investigation, namely working within a structurally dangerous site to physically examine hundreds of tonnes of debris, which was up to four metres in height and heavily compacted.
“We scrutinised every aspect of this fire to ensure our investigation was as robust and thorough as possible.
“Unfortunately, almost everything within the building was severely damaged or consumed in the fire and that included any potential items of evidence that could have provided those answers.”
In a joint statement from Glasgow School of Art, director Penny Macbeth and interim chair Kristen Bennie said: “It has been a very difficult time for the city, and in particular our closest neighbours, for students, staff and the wider heritage sector, all of whom have been impacted in myriad ways by both Mackintosh Building fires.
“While the report is detailed and comprehensive, we share the frustration many will feel that the exact cause has not been identified.
"Taking time to study and consider the report thoroughly, in particular the recommendations, we will now undertake and share a lessons learned exercise on all aspects of the restoration to inform future construction projects.
“The investigation report is an important milestone allowing us to progress the Mackintosh Project as outlined last October.
"We are committed to the faithful reinstatement of the Mackintosh Building within the practical constraints of the regulatory environment, as an integral part of the GSA, as a catalyst and key driver for the social and economic revival of Garnethill and Glasgow and as an exemplar of sustainability for heritage buildings.”
City council leader Susan Aitken said: ““Many people, especially local businesses and residents who were displaced from their premises and homes, will understandably be disappointed that there is not a clearer cut outcome from this long investigation.
"However, it’s clear that Scottish Fire and Rescue Service officers have been extremely diligent in conducting an incredibly complex investigation and I’m grateful to them for their work.”