A turbine blade having labyrinth of internal channels for circulation of coolant, the novel labyrinth geometry includes; inlet arranged on axially upstream face of terminal portion leading to an upstream duct portion having first section adjacent the inlet and a second section, the second section having reduced cross section compared to first section; a first passage intersecting first section and extending through the blade body towards tip of the blade, a proximal end of leading edge passage arranged to capture incoming air flow; a main blade passage intersecting a downstream duct portion, the downstream duct portion arranged in axial alignment with the upstream duct portion but separate from upstream duct portion; and a restrictor passage intersecting with the mid-blade passage and extending towards a mid-blade duct portion, the mid-blade duct portion in axial alignment with the upstream and downstream duct portions and in fluid communication with the upstream duct portion.