A method and system of controlling the locking/unlocking of the network access functions of a terminal including a security processor, like a mobile telephone handset. The terminal is allotted an original public key for verifying the integrity of data loaded into the terminal, a pair of keys associated with the network being generated along with an original approval certificate for the terminal, a locking certificate containing the original approval certificate, the public keys, configuration data and random data, this locking certificate being signed on the basis of the private key associated with the original public key and loaded into the terminal.On entering a user code, the terminal or a function thereof is unlocked after verification of the integrity of the locking certificate and validation of the user code entered.