WorldPay operates in 70 countries, processing credit card payments for 27,000 customers including Vodafone and Sony, and it sent out an email to customers today alerting them to the problems.
The email said: "The integrity and security of our systems and data is in no way compromised. While this type of attack on our system is difficult to prevent and avoid, it is purely a matter of the network capacity being overloaded by the deliberate actions of a third party."
WorldPay spokesman Simon Fletcher told silicon.com that it is a pure denial of service attack and that the systems and data are secure and have not been compromised.
"It is a concerted and orchestrated attack. It is pure 'can we get the Internet to fall over' for the sheer for the hell of it -- and they did," he said. "Our systems are actually functioning but it is just because the pipe is so blocked up with detritus we can't get that information out, or the transactions are very slow compared to what they usually are. Our pipe to the Internet has been blocked."
The reason customers had not been informed until 24 hours after the attack began was because the attack also prevented the firm sending out the emails.
The spokesman said two changes have now been implemented to bring the payment service back to normal. It is re-routing transactions and filtering genuine ones from those generated by the denial of service attack. It could take up to a day to resume normal service.
Another option for customers is a "small" technical change at their end that will point transactions to a back-up WorldPay set-up. For customers choosing this option, service will be restored to normal straight-away, the spokesman said.
He said the immediate priority is getting the WorldPay service running back to usual levels, and then a decision will be made on tracing and investigating the attack.