The invention relates to a method and system for authentication of a user (11) at an access device by means of a cellular mobile radio network (17). The access device gives access to a facility or a service different from the mobile radio network (17). The cellular mobile radio network (17) comprises a base station (12) defining a mobile radio cell (16) with a unique identifier. A mobile radio terminal (14) having a unique identifier books into the mobile radio network (17) via said base station (12), and the authentication is carried out by means of the identifier of the mobile radio cell (16) and the identifier of the mobile radio terminal (14). The base station (12) is arranged at the location of the access device so that the access device has its own mobile radio cell (16) serviced by the base station (12), this mobile radio cell (16) defining an authentication cell (16) of the mobile radio network (17). The mobile radio terminal (14) is caused to book into the mobile radio network (17) via said base station (12) of said authentication cell (16) for the purpose of authentication at the access device whereupon the identifiers are sent to an authentication entity (18) that checks whether the user (11) is permitted to get access at the access device. Access is granted or denied depending on the permission of the user (11).