Five fire engines and a fire rescue unit were called to the scene shortly after 2pm.
Around 30 firefighters tackled the fire and explosion at a workshop on Denmark Hill, close to Coldharbour Lane in Camberwell in South East London.
Around half of the ground floor of the building has been damaged. Three people were taken to hospital by ambulance.
Firefighters from Brixton, Peckham, Old Kent Road and Lewisham attended. The Met Police and London Ambulance Service are also there.