Major credit card companies in the United States will soon begin flagging transactions at firearms shops with a special code that will allow purchases of guns and ammunition to be tracked by law enforcement and other authorities.

Executives at MasterCard, Visa, and American Express said they would implement specifically for stores that sell firearms a “merchant category code” that was approved Friday by a Geneva-based body that manages international financial transactions.

Until now, stores that sell guns were categorized as general merchants or in the sporting goods category. 

The International Organization for Standardization approved a request from Amalgamated Bank of New York to create the new code. Amalgamated said it needed it in order to carry out “socially responsible” investing and lending. The effort was supported by Mayor Adams of New York City and other gun control advocates.

“We all have to do our part to stop gun violence,” the president and CEO of Amalgamated, Priscilla Sims Brown, said. “And it sometimes starts with illegal purchases of guns and ammunition. The new code will allow us to fully comply with our duty to report suspicious activity and illegal gun sales to authorities without blocking or impeding legal gun sales.”

Gun control advocates have for years been pressuring the Geneva body — along with the credit card companies — to create the code. Earlier this month, a group of 40 Democrats in Congress sent a letter to the chief executives of MasterCard, Visa, and American Express saying the code would help law enforcement flag potential mass shooters and gun traffickers.

“Mass shooters have repeatedly financed deadly massacres using credit cards, and Bank CEOs need to step up to save lives,” one of the letter’s signatories, Senator Warren of Massachusetts, said. “Financial institutions and payment networks, such as Visa, MasterCard, and American Express can and should do everything they can to help law enforcement prevent some mass shootings by identifying suspicious gun purchases through the implementation of this new code.”

Gun rights groups, however, said the measure unfairly singles out law-abiding citizens as potentially dangerous merely because they are exercising their constitutional rights. The practical effect, the National Rifle Association said in a statement Sunday, is to create a national registry of gun owners.

The Geneva body’s “decision to create a firearm specific code is nothing more than a capitulation to anti-gun politicians and activists bent on eroding the rights of law-abiding Americans one transaction at a time,” the group said.

Gun rights advocates have argued that the pressure campaign on banks and credit card companies is meant to “privatize” gun control in ways that would be constitutionally suspect if undertaken via legislation.

Credit card companies are already required to use merchant codes to be on the lookout for terrorist financing, money laundering, and other suspicious behavior, but until now gun merchants have not been marked as a class by themselves in the online financial systems.

Merchant codes can track where a customer used a credit card and how much was spent, but not the specific items purchased.