WorldPay, which is owned by the Royal Bank of Scotland, serves over 27,000 businesses throughout the world, from large corporations to small on-line vendors. It operates as a one-stop-shop payment system, allowing retailers to process different payment types and currencies from all over the world.
However, retailers have been badly affected since Saturday after the payment system was brought down by a denial-of-service attack (which involved the WorldPay system being bombarded with false computer-generated requests).
The attack is continuing, and WorldPay is unable to say when its networks will be back to normal, but the company is reported as confirming that the integrity and security of its system has not been compromised.
Speaking to BBC NewsOnline, WorldPay spokesman Simon Fletcher admitted, "We realise it is disrupting our business and our customers business and we apologise unreservedly for that."
WorldPay suffered a similar attack in November 2003, lasting for three days.