Visa and Mastercard have been subject to distributed denial of service (DDoS) attacks by hacker vigilante groups in retaliation for their decision to pull services from Wikileaks.
The groups are angry at commercial organisations boycotting the whistle-blower site following its release of classified diplomatic material.
The action led to Mastercard being unable to process some interactions earlier today.
Hacker group Anonymous has claimed responsibility for the Mastercard DDoS.
In addition, international credit card IT company DataCell, which helps Wikileaks collect payments, is threatening to sue Visa and Mastercard, according to a report on The Guardian.
It has said it will take up "immediate legal actions" to ensure that donations can once again be made to Wikileaks.
"Visa is hurting Wikileaks and DataCell in high figures," it said in a statement. "Putting all payments on hold for seven days or more is one thing but rejecting all further attempts to donate is making the donations impossible."
Similar DDoS attacks were made by Anonymous on PayPal and Swiss Bank Post Finance, after the two companies froze assets belonging to Wikileaks founder Julian Assange.