1. A system for enforcing security policies on mobile communications devices, the mobile communications devices being adapted to be used in a mobile communications network in operative association with a subscriber identity module, the system having a client-server architecture and comprising: a server including a computer operated by a mobile communications network operator; and a client resident on a mobile communications device on which security policies are to be enforced, wherein said server is adapted to: determine security policies to be applied on said mobile communications device; send thereto a security policy to be applied; and send to the client a policy apply message instructing the client to apply a specified security policy already stored in the client, the server comprising a server authentication function adapted to authenticate the security policy to be sent to the mobile communications device; and wherein the mobile communication device, when instructed by said client, is adapted to: receive the security policy to be applied from the server; assess authenticity of the security policy received from the server by exploiting a client authentication function resident on the subscriber identity module; and apply the received security policy; the mobile communication device further comprising: an interface component, which, when the subscriber identity module is executed by the mobile communications device, interacts with the subscriber identity module; the interface component invoking the client authentication function on the subscriber identity module when the client receives the security policy; and the client authentication function resident on the subscriber identity module calculating authentication information on the security policy received from the server; and a manager module, upon receipt from the server of the policy apply message, causes the client to: invoke the client authentication function for assessing an integrity of the identified security policy by exploiting the authentication information; and pass the specified security policy to an enforcer module for applying the specified security policy.