American Express is becoming a much stronger force in the credit card market, according to a report published yesterday.Adam Beu, an analyst with banking industry research group Canstar, said a significant development in the credit card market over the past year has been the increase in merchant acceptance of American Express cards.Canstar's latest credit card reward ratings have American Express cards featuring prominently among the top-rated cards in the survey.  Beu said the increase in merchant acceptance was due, in part, to the influence of the major banks, which all offer companion card arrangements. This means customers can have a Visa or MasterCard credit card and an Amex card operating on the same account.The package appeals to consumers because while Visa and MasterCard have wide acceptance Amex offers higher rewards points.Beu said another factor was that Amex was offering more competitive merchant service deals, especially for online retailers.An American Express spokesman confirmed that the number of Amex cards on issue had increased over the past year and that more merchants were accepting the card. He did not provide any details.The spokesman said Amex has five "strategic alliance partners". These are paid to acquire merchants on Amex's behalf. The partners are ANZ, National Australia Bank, Westpac (and St George), BWA Merchant Services (a subsidiary of First Data Corp) and payment processor eWAY.Amex started signing up merchants through strategic alliances in 2004.