The advent of new and sophisticated web services provided by Service Providers to users, services that individually require authentication of user and authorization of access, brings the needs for a new service to facilitate such authentication and access, a service referred to as Single Sign-On (SSO). The basic principle behind SSO is that users are authenticated once at a particular level, and then access all their subscribed services accepting that level of authentication.The present invention provides a system, method and apparatus wherein a cellular Federation of mobile network operators becomes an SSO authentication authority for subscribers of this Federation accessing Service Providers having such agreement with a mobile network operator of the Federation. In accordance with this invention, mobile network operators can leverage their operator-subscriber trust relationship in order to act as SSO authentication authority for those subscribers accessing Service Providers in a service domain other than the mobile network domain.