A back cover for a mobile phone is described. The cover comprises an elongate body having an outer surface exposed on attachment to the phone and inner surfaces comprising first and second guiding surfaces; and lockable restraining means arranged to co-operate with corresponding restraining means of the phone on the movement of the cover with respect to the phone along said guiding surfaces, thereby preventing substantial movement of the body with respect to the phone in said lengthwise direction. The lockable restraining means of the cover or the corresponding restraining means of the phone comprises a resilient, outwardly flexible part. This part resiliently flexes on said movement of the cover with respect to the phone along said guiding surfaces and forms a user releasable locking engagement between the cover and the phone. The locking engagement is released by the user pulling outwards on the member.