A transaction authorization system (200) includes a client terminal (20) in the form of a computer (22) running a network browser. The computer (22) is connected to the internet (24). The system (200) also includes an account administration server (26) which is connected to the internet (24) and which hosts a web server which may be accessed by the network browser running on the computer (22). The system (200) further includes an account database (30) which stores a record of the status (i.e. disabled or enabled) of an electronic on line bank account. The system (200) is configured to enable a user, via the computer (22), to set the status of the account in the database (30). When a transaction request is issued to a merchant (201) the authorizing institution is able to interrogate the database (30) to determine the status of the account and thereby to authorize or decline the transaction.