Six fire engines and 35 firefighters and officers were called to a fire at a hospital in Rom Valley Way, Romford.
The fire damaged small part of a decontamination room.
The Brigade was called at 0502 and the fire was under control at 1016. Fire crews from Dagenham, Hornchurch, Romford, Harold Hill and surrounding fire stations attended the scene.
The cause of the fire is believed to be accidental and involving electrical equipment.