A three-stage hydraulically-operated telescopic prop is disclosed having working chambers which can be charged with hydraulic pressure fluid to extend a central prop member relative to an outer prop member and an inner prop member relative to the central member. A pressure-operated spring biased non-return valve is operated by pressure to open communication between the working chambers to extend the inner member once the central member has been fully extended. The valve is also operated mechanically to open communication between the working chambers to allow retraction of the inner member when the central member has been retracted to a certain extent. Further chambers are provided between the central and outer member, and between the central and inner member, and these chambers can be charged with hydraulic pressure fluid to retract the central member relative to the outer member and the inner member relative to the central member. Guide bushings with sealing and/or packing rings are secured to the upper ends of the outer and central members to locate and guide the central and inner members during their movement.