A support prop which includes telescopically interengaged inner and outer tubular members, a one-way valve for introducing pressurized liquid into a volume inside at least one of the members, a pressure relief valve for releasing liquid from the volume when the liquid pressure in the volume exceeds a predetermined value and a number of wedges which are located between opposing surfaces of the tubular members and which are engaged with one member and which deform the inner member when telescopic yielding action of the members takes place.