An actuator comprises a ball nut and a lock housing arranged radially outwardly of the ball nut and overlapping a proximal end of the ball nut when the ball nut is in a stowed position. The lock housing comprises a sleeve in which is located an aperture, and a locking element is retained within the aperture and can move in a radial direction through the aperture. When the ball nut is in the stowed position, the locking element engages with a locking projection on the ball nut, to retain the ball nut in the stowed position.