Visa has suspended all payments to WikiLeaks in the latest blow to the website business.
The card payment operator said an inquiry is under way into how the organisation operates.
The whistle-blowing website has suffered repeated denial of service attacks, moved server, lost its PayPal service and had a key Swiss bank account closed.
WikiLeaks relies on online donations from a worldwide network of supporters to fund its work.
A spokesman said: "Visa Europe has taken action to suspend Visa payment acceptance on WikiLeaks' website pending further investigation into the nature of its business and whether it contravenes Visa operating rules."
And fellow payments giant MasterCard said it is "currently in the process of working to suspend the acceptance of MasterCard cards on WikiLeaks".
On Monday, the Swiss post office's bank, PostFinance, shut accounts opened by Mr Assange containing a defence fund and personal cash of 31,000 euro.
On Saturday it emerged online payments processor PayPal had cut access for donations to WikiLeaks.
The company said its payment service cannot be used for activities "that encourage, promote, facilitate or instruct others to engage in illegal activity".
The company providing WikiLeaks with its domain name, EveryDNS.net, also cut off service because the domain wikileaks.org was repeatedly attacked.