The internet payment system Worldpay is under hack attack from unknown assailants, hitting thousands of online retailers around the world.
The company's payment and administration networks have been flooded with computer-generated requests, clogging the system and slowing transactions - also known as a "denial-of-service" attack.
Worldpay is owned by the Royal Bank of Scotland Group. It has 27,000 clients around the world, ranging from heavyweights like Vodafone and Sony Music Entertainment to numerous small online retailers.
The company, which operates in more than 70 countries stretching from Australia to Sweden and accepts payments on credit cards including Visa, Mastercard, Diners and American Express, said the bombardment started Tuesday and is still going on.
Worldpay has beefed up its security measures and the payment and administration systems have not been compromised, said Carolyn McAdam, a company spokeswoman.
"We have been working flat out and have been since the incident started," Ms McAdam told BBC News Online. "We hope to have it sorted out in the course of today."
This sort of skirmish is nothing new in cyberspace.
In September, net provider PSINet and security firm PanSec International said an unprotected Web site they set up as part of a study was attacked about 2,000 times a week over a two month period.
At the start of this year, Visa and Mastercard admitted that a hacker had gained access to more than 5 million of credit card accounts.