A method of and system for authorizing purchases made over a computer network is provided. In accordance with the present invention, a consumer electronically transmits an ATM card number over the network to an on-line merchant from which the consumer desires to make a purchase. The on-line merchant then electronically forwards the ATM card number to a third party contractor, such as a bank, that will oversee and authorize the transaction. The third party contractor subsequently determines an authentication token type associated with the card and electronically prompts the consumer for the appropriate type of authentication token, such as a PIN or biometric signature or the like. The consumer then inputs and electronically transmits the authentication token over the network to the third party contractor, bypassing the on-line merchant. Having both the ATM card number and the authentication token, the third party contractor verifies that the ATM card number and authentication token are valid, checks for sufficiency of funds, and either authorizes or denies the transaction. The authorization or denial is communicated to the on-line merchant over the network, who either completes or rejects the purchase.