An instrument (200) for positioning a hollow cup component (202) of an orthopaedic joint prosthesis and assembly thereof are described. The instrument includes a support member (204) having a plurality of jaw members (208) unitarily formed around its periphery. Each of the jaw members has a cup engaging limb (210) extending in a first direction to be received in a recess on an external side wall of the cup component. Each jaw member is pivotable between an engaged position in which the cup engaging limbs are displaced inwardly and a disengaged position in which the cup engaging limbs are displaced outwardly allowing the cup component to be removed from between the jaw members. An actuator (216) is operable to adopt an engaged configuration in which the actuator acts on all the jaw members to urge them into the engaged position or a disengaged configuration in which the actuator does not act on any of the jaw members to urge them into the engaged position and all the jaw members can adopt the disengaged position.