Method for securely authenticating a roaming terminal, wherein: - the terminal is able to connect with a mobile telecommunications network by means of a mobile telecommunications modem comprising an USIM card, - the terminal connects to an applications server, of a wireless telecommunications network, through a wireless access terminal, following an authentication procedure of the USIM card by an authentication server according to an EAP authentication protocol, characterised in that, during the authentication procedure, a dialog session is initiated between the terminal and the USIM card, according to a communications protocol secured with controls and responses having a first predetermined sequence of the AT+CSIM type; in that an AT-CSIM application for the sequencing of the communication and an EAP-SIM application are downloaded at the same hierarchical level as the USIM application, said applications being separate from one another and the AT-CSIM application and the EAP-SIM application executing in parallel to the USIM application; in that a communication sequencing programme for the communication between the USIM card and the terminal is installed on the terminal, said programme being a mutual exclusion algorithm Mutex ("Mutual Exclusion") and a sequencing protocol designed to prevent the resources of the USIM card, which are shared by multiple applications, from being used simultaneously, said programme allowing to regulate access to the USIM card data, so that the AT-CSIM application and the EAP-SIM application are the only ones that can be executed, said programme using a state to control the execution of the AT-CSIM application, in that so long as that state is used, the USIM card is considered as in use for all the other controls, and in that as soon as the state is no longer used, said programme considers that the AT-CSIM application is completed; in that said programme allows to temporarily block the AT controls used by the USIM application for its standard function by privileging the AT-CSIM controls required for the EAP-SIM application, and allows to avoid possible dysfunction and disruption during the dialog session, said blockage only lasting a few milliseconds to influence the function of the USIM application managing the connection to the mobile network; in that, during powering up by the terminal, a USIM session is initialised, and in that in a USIM session, an Application Dedicated File, ADF, application, among those present on the USIM card, is selected by default during initialisation, said ADF application being the one used by the USIM on channel 00; and in that, to prevent any conflict with the USIM application, the AT-CSIM and EAP-SIM applications use another channel than the base channel 00, these other channels can be logical channels 01, 02 or 03. - the terminal is able to connect with a mobile telecommunications network by means of a mobile telecommunications modem comprising an USIM card, - the terminal connects to an applications server, of a wireless telecommunications network, through a wireless access terminal, following an authentication procedure of the USIM card by an authentication server according to an EAP authentication protocol,