A thermal control system for a turbine casing (2) mounted radially outwardly of and circumferentially surrounding blade tips of the turbine blades, the system comprising: one or more circumferential chamber walls (6), each disposed around the casing (2), each to form a circumferential flow chamber (7) radially outward of said surface of the turbine casing (2), each to form a circumferential gap (12) between the circumferential chamber wall (6) concerned and said surface of the turbine casing (2), and the one or more circumferential chamber walls (6) define at least one opening (9) on the radially inward side of the chamber (7) adjacent the surface of the turbine casing (2) such that a thermal fluid injected into the flow chamber (7) concerned may leak out through the circumferential gap (12) via the opening (9), and means to induce circumferential flow on the surface of the casing (2) bounded by the circumferential flow chamber (7) comprising one or more ducts (8) radially outwardly connected to the or each circumferential flow chamber (7), via which the thermal fluid can be injected into the flow chamber or chambers (7) concerned in a circumferential direction.