The fire, which began at 8.30am in the hospital’s fifth floor cardiac wing, is believed to have been caused by an exploding gas cylinder.
A hospital spokesperson said patients, relatives, and staff had all been safely evacuated and there were no reported injuries.
The London Fire Brigade said six fire engines and 35 fire-fighters were called to attend the blaze.
London Ambulance Service has confirmed to NT that four fire fighters were taken to hospital with minor injuries as a result of attending the fire.
Brian Bell-Fortune, Great Ormond Street Hospital for Children NHS Trust student practice facilitator, said: ‘Everyone was quite sensible, they took it in their stride and were very calm.
‘Nurses in and out of uniform helped to evacuate the wing. It is an unusual situation, but the hospital staff reacted well.’
Did you help to evacuate patients? Call the NT news desk on 020 7728 3716.