American Express faces further declines in market share this year as Commonwealth Bank prepares to stop marketing AMEX-branded companion cards in November.CBA is the last of the major Australian banks to abandon a companion card deal with AMEX following the move last July by the Reserve Bank to impose interchange fee caps on credit card transactions.The caps have forced the major banks to reduce earn rates on rewards programs and rationalise their card offerings to preserve margins.The regulatory move has had a profound impact on AMEX's combined share of the credit and charge card markets, according to official data published by the RBA's Payments System Board. AMEX accounted for more than 19 per cent of payments made on credit and charge cards in Australia in May last year but that has fallen to 17 per cent as each of the major banks have culled the companion card.ANZ, Westpac and NAB have already binned their companion card deals, but AMEX recently entered a new distribution arrangement with Westpac that apparently circumvents the regulatory caps.Westpac now markets two new AMEX cards to its customers that are issued and managed by the global card company, rather than the bank.This means that the RBA's interchange caps do not apply to the new AMEX cards being marketed by Westpac.AMEX vice president Jennifer Berthold said the RBA's changes had a disruptive impact on the payments industry."The RBA's regulatory changes introduced in July last year created widespread disruption of the payments landscape with banks choosing to either devalue their rewards programs or remove the American Express companion cards," she said."With our proprietary card business not included in the regulation, it was important for us to leverage this advantage."Berthold appeared to question the RBA's market share data, saying that AMEX was winning new customers."We're seeing considerable growth in our proprietary cards business in Australia, as more and more customers have chosen to come us directly for an American Express card," she said."We're significantly outpacing the market on year-on-year growth on key metrics such as new accounts, average spend and balance growth."