The group of activists, calling themselves Anonymous and linked to the 4chan site are understood to have targeted 'anti WikiLeaks' sites in a move dubbed 'Operation Payback'.

In a tweet the group claims to have caused an outage today, ongoing at the time of writing, at mastercard.com. This follows a similar distributed denial of service attack that forced PayPal's blog offline. The Web site of Swiss bank Postfinance was also recently taken down for around 24 hours after it closed WikiLeaks founder Julian Assange's account.

On Monday MasterCard said it is blocking payments because WikiLeaks is engaging in illegal activity, a move soon followed by Visa, as corporate America joins in the condemnation of the organisation as it continues to embarrass the US political elite by publishing leaked diplomatic cables.

In a statement today, the firm admitted it is "experiencing heavy traffic on its external corporate website" but claimed it "remains accessible" and initally insisted that "there is no impact whatsoever on our cardholders' ability to use their card for secure transactions".

However this has been contradicted by the BBC's technology correspondent Rory Cellan-Jones on Twitter: "Paypoint says all payment services companies have been affected by an outage at MasterCard's SecureCode service.."

In a new statement, quoted by the Guardian, MasterCard is now admitting issues with payments: "Please be advised that MasterCard SecureCode Support has detected a service disruption to the MasterCard Directory Server. The Directory Server service has been failed over to a secondary site however customers may still be experiencing intermittent connectivity issues. More information on the estimated time of recovery will be shared in due course."

Meanwhile, DataCell, the Iceland-based firm that enables Wikileaks to accept card payments says it will take immediate legal action against Visa Europe over the issue.

In a statement, DataCell CEO Andreas Fink says he has received a suspension notice stating that Visa Europe has ordered the payment processor to suspend payments for one week from today.

DataCell has "decided to take up immediate legal actions to make donations possible again"

Fink goes on: "The suspension of payments towards Wikileaks is a violation of the agreements with their customers. Visa users have explicitly expressed their will to send their donations to Wikileaks and Visa is not fulfilling this wish. It will probably hurt their brand much much more to block payments towards Wikileaks than to have them occur. Visa customers are contacting us in masses to confirm that they really donate and they are not happy about Visa rejecting them. It is obvious that Visa is under political pressure to close us down."

PayPal originally said that the WikiLeaks' account had violated its terms of services when it stopped processing payments on its behalf last week. The firm's Osama Bedier has now told the Le Web conference in France that the decision came off the back of a State Department letter asserting that WikiLeaks' activities were illegal.

State Department spokesman PJ Crowley has tweeted: "The U.S. government did not write to PayPal requesting any action regarding #WikiLeaks. Not true."