Embodiments of the invention provide a method of processing payment authorization requests for payment transactions to be conducted via a data communications network on behalf of online merchants, the payment authorization requests are conducted as a result of orders by financial instrument holders via a plurality of different online merchant systems. Each of the online merchants has an online merchant identity. The method is conducted by a trusted central intermediary system which is arranged to transmit payment authorization requests to each of a plurality of different online merchant Internet Payment Service Provider (IPSP) systems. Each merchant IPSP system is arranged to transmit payment authorization requests to at least one of a plurality of acquiring bank payment processor systems, and each of said plurality of acquiring bank payment processor systems is responsible for processing payment authorizations for at least one of said acquiring banks. Embodiments of the invention enable a user to select a payment method on a per transaction basis, while removing the requirement for the user to provide payment details to individual online merchant systems or to their merchant IPSP systems. Thus, providing that online merchants, or their merchant IPSPs, subscribe to a service arranged to perform the method, users only have to submit their respective payment details, preferably only once, to a separate, trusted entity.