Abstract: A transaction processing system for enabling users to authorize transactions, comprising: at least a first data communications interface and a second data communications interface; a data receiver arranged to receive transaction data, relating to a transaction to be authorized by a user, via a first data communication path, at said first data communications interface; a communications controller arranged to conduct communications over a second data communication path, different from said first data communication path, with said user at said second data communications interface; a data authentication system arranged to conduct a secure access procedure, via said second path, in which authentication data is received and said authentication data is verified, wherein the communications controller is arranged to receive: a first transaction reference relating to said transaction from said user, said first transaction reference not having been previously transmitted to said user via said second communication pa