a collapsible steering column assembly comprising a mounting structure , the mounting structure comprising a first part for connecting to the body of a vehicle and a second part for connecting to part of a steering mechanism , the first and second parts being interconnected to permit relative movement therebetween as the assembly collapses , wherein : one of the first or second parts comprises a slot , the slot comprising a pocket and a channel defined by two opposing sidewalls , each sidewall being the outer edge of a deformable structure ; the other of the first or second parts comprises a lug , the lug extending into the pocket when the assembly is in a non - collapsed state and being configured to be driven through the channel to cause plastic deformation of the deformable structure as the assembly collapses ; and the assembly comprises a re - enforcement that limits the plastic deformation of the deformable structure during the collapsing stroke to regions of the deformable structure adjacent to regions of the channel through which the lug has passed .