Four fire engines and 21 firefighters and officers on a building site near Kings Cross Station on Goods Way.
Foam insulation placed into an excavated hole was on fire. The smoke travelled through nearby tunnels into Kings Cross Station setting off their fire alarms.
The Brigade was called at 0020 and the fire was under control by 0133. Fire crews from Holloway, Kentish Town and Soho fire stations attended the scene. The cause of the fire is under investigation.