Between noon and 2pm UK time, visitors to the MasterCard Website were timed out, indicating a massive distributed denial of service attack on the card group.

The broadside opens up a new front in the ongoing battle between underground hacking rings and large corporate interests.

The latest assault comes six months after hackers, loosely organised under the 'Anonymous' umbrella, successfully disrupted the Internet operations of a clutch of international payments groups in retaliation for cutting off payments to WikiLeaks.

More recently, the underground movement has appeared more concerned with exposing defective security precautions on corporate sites, dumping account names and other data on the public Web. The strike against MasterCard comes just a week after now-disbanded hacking collective LulzSec signalled a change in direction, announcing plans to partner with Anonymous on a new campaign dubbed Operation Anti-Security, targeting banks and governmental agencies.

The humbling of MasterCard was posted on Twitter by WikiLeaks, with an accompanying statement which named Visa, MasterCard, PayPal, Bank of America, and Western Union as the chief 'culprits' in an 'unlawful banking blockade' against the company.

WikiLeaks says the strangling of fund raising has so far cost it $15 million in lost revenue, and is urging supporters to use the alternative currency Bitcoin to make donations.

"When Visa and MasterCard will happily provide services to the Klu Klux Klan, but not to WikiLeaks, it is time to act," says the organisation.

What Does it Cost to Change the World? from WikiLeaks on Vimeo.

Update: In a carefully-worded statement filed later in the day, MasterCard blamed the downtime on a server outage, but stopped short of an outright denial that this was caused by a DDoS blitz: "We can confirm that MasterCard's corporate, public-facing website experienced intermittent service disruption, due to a telecommunications/Internet Service Provider outage that impacted multiple users. It is important to note that no cardholder data has been impacted and that cardholders can continue to use their cards securely. We are continuing to monitor the situation closely."