Abstract: An authentication system (10) allows the identity of a user (12) to be authenticated when the user (12) is seeking access to a secure service provided by a server (14). The system (10) comprises two separate communications channels. The first channel is a network (20) for allowing the user (12) to communicate with the server (14). The second channel is a mobile communications channel (26) that utilises a mobile communications device (28) for allowing an authentication server (22) to communicate with the user (12). In use when the user (12) requests access to the server (14), he or she sends a username to the server (14). The server (14) generates a request for the confirmation of the user's identity, which it sends to the authentication server (22). The authentication server (22) in turn generates a passcode and also queries a user database for the mobile communication device network number of the user (12).