The Fire Brigades Union says five firefighters received treatment after tackling the blaze in Edinburgh.
A firefighter has been seriously injured in a blaze at the Jenners building in Edinburgh.
Fire crews, police and ambulance staff were dispatched to the former department store on Rose Street at about 11:30.
The Scottish Fire and Rescue Service sent 22 fire engines and said the fire was well-established when they arrived.
Eyewitnesses have described smoke pouring out of the basement area of the department store.
The Fire Brigades Union said it had been made aware of the reports of injuries to firefighters during the incident at Jenners.
A spokesperson said: “We can confirm that five firefighters have been treated at the scene or at hospital, one of whom is in a serious condition.”
No casualties have been officially reported.
Eyewitnesses have described smoke pouring out of the basement area of the department store after the blaze was discovered.
A police cordon was extended to cover the whole of St Andrew Square, and staff from nearby offices were evacuated.
Eyewitnesses told BBC Scotland about the response of emergency services on Princes Street.
Sarah Mullins, 34, manager at Wagamama in St Andrew Square, said she first saw smoke at about 11:30.
“It got so bad we couldn’t see out across the square,” she said.
“The police told us to stay inside because the smoke was so bad it would have affected our lungs. They said it was safer to stay inside.
“It’s very sad this has happened to such an iconic building.”
Founded in 1838, the Jenners building was one of the oldest department stores in the world until it closed.
It has been undergoing a restoration which was due to take four years. Under the plan, disused rooms in the six-storey building are due to be made into a hotel.
The current building was designed by architect William Hamilton Beattie in the Victorian renaissance revival style and opened in 1895 – after the original building was destroyed by fire in 1892.
The building was sold to private investors in 2005 after House of Fraser bought the Jenners brand and property.
It was then bought by Danish billionaire Anders Holch Povlsen in 2017 for a reported £53m.