A method of authenticating a user who is a subscriber of a home network, authenticated in a first network, for accessing a service in a second network. This method includes: authenticating the user in the first network with a first authentication method selected in an authentication server; reserving resources for the service towards a rules enforcement device; requesting control rules for the resources towards a control rules server; submitting towards the control rules server information about the first authentication method; determining at the control rules server whether a further authentication of the user with a further authentication method is required; and instructing from the control rules server towards the authentication server to force the further authentication of the user with the further authentication method.