A support prop which includes a cylindrical aluminium liner which is externally reinforced with a shell made from a composite material, an aluminium plunger with a seal which engages with an inner surface of the liner, a filler valve through which a volume inside the liner, adjacent the seal, in use is pressurized with a fluid, and a pressure relief valve which in use allows fluid under pressure to escape from the volume as the plunger is forced into the liner.