The Durbin amendment, which was passed last week, would give the Federal Reserve the power to regulate debit interchange fees and free up merchants to offer alternative forms of payment.

"It appears that in an effort to frighten small banks and credit unions into opposing the amendment, your companies are threatening to make changes to your small bank interchange fee rates and to your network operating rules," Durbin wrote. "These changes, which are not in any way required by the amendment, are unnecessary and would disadvantage small card-issuing institutions … The simple fact is that small banks would not be harmed or punished under the amendment unless your companies decide to harm or punish them."

Durbin warned the card schemes that any co-ordinated attempt to change fees and rules in the wake of the amendment "would raise serious concerns that you are engaging in an unlawful restraint of trade".