A gas turbine engine casing 2 comprising: an inner circumferential wall 12; an outer circumferential wall 14 spaced radially outwardly from the inner wall 12; wherein the inner and outer circumferential walls 12, 14 are formed by an axially repeating profile comprising an inner wall portion 6 and an outer wall portion 8 connected to one another by an intermediate portion 10, the axially repeating profile being arranged such that the inner wall portion 6 abuts against and is connected to an adjacent inner wall portion 6 to form the inner circumferential wall 12 and the outer wall portion 8 abuts against and is connected to an adjacent outer wall portion 8 to form the outer circumferential wall 14.