The Reserve Bank has moved to include American Express companion cards in its regulation of card payments.The RBA announced last week that Amex companion cards, along with prepaid cards issued by eftpos, MasterCard and Visa and MasterCard debit cards would fall under payment system regulation.Under companion card arrangements, card issuers offer a Visa or MasterCard credit card and an Amex card operating on the same account. The package appeals to consumers because Visa and MasterCard have wide acceptance, while Amex offers higher reward points.The RBA's regulation involves the setting of benchmark rates for credit and debit card interchange fees. Up to now, Amex has operated outside the regulatory system - something that has prompted plenty of complaints from rival card schemes.The RBA said in a statement: "Designation does not impose regulation; rather it is the first of a number of steps the bank must take to exercise any of its regulatory powers."The RBA's move follows the finding of the Financial System Inquiry that differences in the structure of payment systems had resulted in systems that perform similar functions being regulated differently, which may not be competitively neutral. The FSI said: "The Inquiry considers that payment systems of similar economic substance should be regulated consistently."MasterCard issued a statement on Friday asking why other parts of Amex's business were not being designated and why other payment schemes (such as PayPal) were not being designated."This means that Australia looks set to continue having an unequal playing field in payments," MasterCard said. The RBA's Payments System Board has been reviewing regulatory arrangements applying to the payment card system this year. In March it published an issues paper.