Three people had to be lead to safety from their flats after a fire was deliberately started in a staircase.
Firefighters were called to the property in High Steet, Lancaster, at 10.17pm on Monday and found a concrete staircase well alight on arrival.
Watch manager John Timperley said: “There were lots of combustables - things like pieces of wood and other things that shouldn’t be there - and that was the only way out of the flats.
“Smoke was seeping into flats on the ground, first and second floors, and the occupants were trapped inside.”
Firegfighters using breathing apparatus and hose reels tackled the fire before they could reach the stricken people and lead them outside.
Mr Timperley added: “Luckily everyone was ok apart from some smoke inhalation, but this had the potential to be very serious.
“The police are now working with us on an investigation into what has happened.”