AN investigation into the cause of the second Glasgow School of Art fire will not be completed until more than three years after the blaze.

The investigation was condemned as having “dragged on for too long now”.

In a letter to the Scottish Parliament committee investigating the devastating inferno, the Scottish Fire and Rescue Service (SFRS) said: “The fifth phase of the forensic examination involves the remainder of the building, but mainly located to the areas facing Renfrew Street. An approximate time scale for the clearance of the remainder of the GSA is late August 2021.”

But Pauline McNeill, a Labour candidate for Glasgow, said: “We were told in June 2019 that the investigation was moving into the final stages but still we are no closer to knowing what happened.

“Three years is a long time to wait for answers. Residents, students and those who helped foot the bill for repairs deserve to know the truth so that Glasgow never has to face a disaster on this scale again.”

An inferno had already torn through the prestigious art school four years earlier at the time of the second blaze in 2018.

The site was being managed by Kier Constructions at the time of the second fire.

A huge investigation has been undertaken by SFRS to determine the cause of the blaze, which is still not known.

Fire investigators have worked with GSA staff to clear debris from the building and have even carried out “archaeological sifting” with the help of specialist contractors to salvage artefacts from the Mackintosh Building.

The city centre landmark, designed by Charles Rennie Mackintosh, was considered an architectural masterpiece and holds Grade A-listed status.

Firefighters were called at 11.19pm on Friday, June 15, 2018, when it went up in flames. The nearby O2 ABC on Sauchiehall Street was also consumed in the fire.

A spokeswoman for GSA said: “The SFRS has been clear that the investigation into the cause of the 2018 fire, when the building was in the day-to-day control of Kier Construction (Scotland) Ltd, is very complex.

“We continue to support the SFRS team by making sure that they have full access to any areas of the building they wish to examine as part of their ongoing investigation.”

A Kier spokeswoman said the company has assisted the fire service with its investigation and would continue to do so.

Stuart Stevens, Assistant Chief Officer for Scottish Fire and Rescue's Service Director of Service Delivery, said: “The Scottish Fire and Rescue Service investigation into the 2018 fire at the Glasgow School of Art remains on-going.

“It is not possible at present to provide a definitive timescale for the investigation being concluded, and we would like to thank everyone for their understanding and patience as we continue with this significant fire investigation.”