Six fire engines and around 40 firefighters responded to a fire on Havering Road, Bower Park.
The fire was located in a gymnastics centre, adjacent to a school. The gymnastic centre and a number of porta cabins were destroyed by the fire. The neighbouring school buildings were unaffected, and there are no reports of any injuries.
The Brigade was first called about the incident at 0655, with crews from Romford, Harold Hill and Hainault fire station attended the scene.
The cause of fire has been ruled as an undetermined electrical event involving the buildings electrical intake and distribution board.