The settlement agreed in July was designed to end a seven-year legal dispute over interchange fees, with retailers securing a multi-billion dollar cash payment and significant reforms of Visa and MasterCard rules and business practices.

However, two of America's biggest merchants, Walmart and Target, are among those to have called for the deal to be rejected and now the NRF, the world's largest retail trade association, has entered the fray.

The NRF is not party to the original lawsuit but says that a resolution approved by its board authorises it to take steps including "intervention in pending actions" in order to reach an agreement "equitable to the broad merchant community".

The association says it is looking into what form any legal action might take and is waiting for US District Court Judge John Gleeson to outline how outside groups will be allowed to intervene, or if the case qualifies as a class action.

Combined credit and debit card swipe fees tripled over the past decade to about $50 billion a year - driving up prices an estimated $427 for the average household - before debit swipe was capped by the Federal Reserve last year, claims the NRF.

It says that the $7.25 billion settlement is "pennies on the dollar" and that if the case went to trial a judgement could be worth hundreds of billions of dollars.

The group also argues that the deal does nothing to block future swipe fee rises, reform the "cartel-like" system by which Visa and MasterCard each set fee schedules, or improve transparency.

Matthew Shay, president and CEO, NRF, says the settlement "does nothing to curb the anticompetitive behaviour of Visa and MasterCard, and instead ensures that swipe fees paid by retailers and their customers will continue to rise while barring any future legal challenges. The proposal is a lose-lose-lose for merchants, consumers and competition."