The new merchant category code (MCC) is designed to show where firearm purchases are made but not details on what has been bought.

The card giants agreed to implement the code last year after the International Organisation for Standardisation approved it in response to pressure from gun-control activists, who say it will help track suspicious weapon purchases.

However, there has been significant opposition to the plan, with bills introduced in several Republican-led states that would bar or limit the use of the voluntary code.

In a statement to Reuters, Visa says: "There is now significant confusion and legal uncertainty in the payments ecosystem, and the state actions disrupt the intent of global standards. Accordingly, Visa is pausing implementation of the MCC."

Seth Eisen, SVP, communications, Mastercard, tells Fox: "Today, there are bills advancing in several states related to the use of this new code. If passed, the result will be an inconsistency in how this ISO standard could be applied by merchants, issuers, acquirers and networks.

"It’s for that reason that we have decided to pause work on the implementation of the firearms-specific MCC."