A fire took hold at a university campus in north-west London for the second time in 48 hours.
About 100 firefighters were at the scene of the fire at the University of Westminster, in Harrow on Sunday night.
The brigade said there were no reports of casualties but 30% of a three-floor classroom block was damaged.
It comes after a hazard zone was set up at the Watford Road campus on Friday night during a fire involving a number of gas cylinders.
Fire crews spent Saturday cooling the cylinders and damping down.
The second fire was brought under control by 0200 BST on Monday.