Four fire engines and around 25 firefighters were called to a fire at a hospital on Rowland Hill Street in Hampstead.
A small part of an office on the ground floor of the building was damaged by the fire. There were no reports of any injuries.
The Brigade was called at 0347 and the fire was under control by 0448. Fire crews from West Hampstead and Kentish Town fire stations attended.
The cause of the fire is under investigation.