Nottinghamshire Fire and Rescue Service has since put out the fire, of which six appliances were at the scene on Victoria Square.
Crews were called to the derelict building at about 5.20am, Friday, June 10.
At 12.03pm the fire service announced the fire had been put out and an investigation into the cause was launched.
A spokesperson said: “The fire is out an investigation into the cause is taking place.
"Residents in Portland Street are asked to keep any windows and doors facing the square closed.
"Portland Street, Dock Road and Gateford Road car park remain closed.
1. Worksop fire
The site has been cordoned off as firefighters tackle the blaze. Photo: Brian Eyre
2. Worksop fire
Six fire engines were dispatched to the scene this morning. Photo: Brian Eyre
3. Worksop fire
The fire has taken place inside a derelict building in Victoria Sqaure. Photo: Brian Eyre