A metal casing strip for a fire collar to be arranged around a duct has a line of succesive tongues and cutouts in the longitudinal direction of the strip. The tongues and cutouts are arranged in the strip in an alternating fashion. The tongues are bendable out of the surface of the casing strip such that in use, when the casing strip is arranged around a pipe such that two end regions overlap, an outwardly bended tongue from the one end region is inserted in a cutout of the overlapping other end region and folded over the edge of the cutout to retain the casing strip in the closed collar shape.