An automatic mechanical lock, particularly for a vehicle door or doors comprises two body portions, one body portion to be mounted on the door at an edge thereof and the other body portion to be mounted adjacent the door and overlapping the edge of the door, a first of the body portions carrying a hasp (12) to receive a spring-mounted bolt (22), and the second body portion (20) carrying the spring-mounted bolt (22) and having an aperture (25) to receive the hasp (12), the bolt (22) being biased by the spring (23) to a position where it can engage the hasp (12) to lock the door, a key mechanism (32, 32A), which when the key is turned in the lock from a first position to a second position, slides the bolt (22) away from the hasp (12) against the action of the spring (23) whereby the door may be opened, an arm (27) movable to engage a slot (24) in the bolt (22) when the key is returned from the second position to the first position to prevent the bolt (22) re-engaging the hasp (12), whereby the door can still be opened, and an actuator (29) spring-biased (30) to block the passage of the bolt (22) when the hasp (12) is removed on opening the door and to allow the hasp (12) to re-enter the aperture (25) on closing of the door, the actuator (29) releasing the arm from the slot in the bolt when the actuator moves to block to the passage of the bolt, whereby when the door is closed, the hasp (12) moves the actuator (29) from its blocking position and the bolt is sprung through the hasp (12).