A total of six fire engines and crews scrambled to the incident at the energy company’s building in 4 Penner Road at 5.01pm today.
The roof of the building is described as being ‘well alight’, according to Havant crew manager Gary Tyler, who remains manning the fire station.
The firefighter said: ‘There is a command support vehicle and an aerial ladder platform also at the scene.
‘If anyone in the area smells smoke, please close your windows and your doors.’
Fire crews have been deployed from Fareham, Hayling Island, Cosham, Southsea, and Emsworth.
More information will be published as details become available.