A turbofan gas turbine engine (10) comprises a compressor rotor (24) carrying a plurality of circumferentially spaced radially extending rotor blades (26) and a casing (28) surrounding the compressor rotor (24) and compressor rotor blades (26). A compressor rotor blade tip seal (48) comprises an annular member (30) secured to and arranged within the casing (28). The annular member (40) has a plurality of circumferentially spaced axially extending corrugations (32) and a plurality of radially spaced circumferentially extending corrugations (34). A lining (40) is secured to and arranged within the annular member (30). The lining (40) is spaced radially from the tips (27) of the compressor rotor blades (26) to form a clearance (29). An actuator (56) is provided to move the annular member (30) and lining (40) radially to control the size of the clearance (29).