A spokesperson for Norfolk Constabulary said that two people had died but could not confirm the cause of their deaths.
He said: "Initial reports from the scene suggested an explosion had taken place.
"The men, one aged in his 50s and one in his 20s, both thought to work at the premises involved, died as the result of the incident shortly after 9am."
Scott Norman, from Norfolk Fire and Rescue Service, said crews arrived at 9.06am.
"When we arrived smoke was billowing from a large building, our crew entered the property and located the room which the fire was in," he added.
"We then extinguished the fire but unfortunately two people died at the scene."
Emergency services at the scene of the fire. Photo: Ashley Cashfield
The accident was believed to have happened at a warehouse belonging to Harford Attachments, a business making parts for diggers.
Ambulances and a helicopter from the East Anglian Air Ambulance was deployed but the crew did not include the Duke of Cambridge, who began work with the charity today.
The East of England Ambulance Service Trust said the two men were pronounced dead at the scene and three other people were treated for smoke inhalation.
Smoke was sent billowing over Norwich by the blaze. Photo: Ashley Cashfield
Photos captured by Norwich resident Ashley Cashfield showed clouds of black smoke rising over the industrial estate as emergency vehicles gathered outside.
An investigation has been launched by the police and fire service to establish the cause of the explosion.