Fire crews were called to former department store Jenners, which was being turned into a hotel, at 11.29am on Monday. Five firefighters were taken to hospital for treatment, with one in a “critical condition”.
Witnesses saw a soot-covered firefighter being helped from the building by colleagues.
Speaking to the BBC on Tuesday, Edinburgh City Council leader Cammy Day said: “The Scottish Fire and Rescue Service along with the surveying team were looking into that overnight, so we don’t yet know the cause.
“Obviously there was construction work happening on the old Jenners building, so we’ll find out in the next coming days.”
Mr Day added: “I sure I say this on behalf of the whole city, that our thoughts are with the firefighters who risked their lives to save the building and save the people around the city as well, so our thoughts are with their families today.”
The council will assess the building’s structural integrity, Mr Day said, adding: “That is a question we don’t have the answer to yet.”
Asked if the building, which has stood in Princes Street since the 1800s, will survive, Mr Day was unable to be definitive.
“That will be subject to these surveys – I’m hopeful that where we saw the fire yesterday was to the rear of the building and I hope that can be salvaged,” he said.
Scotland’s community safety minister Elena Whitham paid tribute to the firefighters who tackled the blaze, saying: “Their sheer bravery and professionalism must be commended. Scotland’s communities are lucky to have our valued firefighters, who stand ready to put their personal safety at risk in order to save others. We will continue to monitor the situation closely.”
In a statement released in the hours after the blaze, the fire service’s interim chief officer Ross Haggart said: “The fire at the Jenners building in Edinburgh is a very serious and complex incident and regrettably I can confirm that five of our colleagues have been taken to hospital for treatment and one remains in a critical condition.”