The internet payment system Worldpay is under attack from unknown hackers, disrupting thousands of online retailers around the world.
Worldpay handles credit and debit card payments
Worldpay's payment and administration networks have been flooded with computer-generated requests which clog its system and slow transactions.
The firm said it was not able to say when the problems, which surfaced on Saturday, would be sorted out.
Worldpay suffered a similar "denial of service" attack in November last year.
DDoS ATTACK EXPLAINED
DDoS = Distributed Denial of Service attack
Malicious hacker uses virus to hijack numerous computers
On command these 'zombie computers' flood the targeted website with useless data
The target's internet servers are overwhelmed by junk data
Customers have trouble using the targeted website
Targeted website can be slow or inaccessible for days
Fighting DoS attacks is laborious and costly
Because the 'zombies' are 'distributed' across the internet, finding the attacker is difficult
Worldpay said its service to customers had been "adversely affected".
However, the firm stressed that transactions were still being processed - albeit more slowly than usual - and that the security of data had not been compromised.
"We realise it is disrupting our business and our customers' business and we apologise unreservedly for that," spokesman Simon Fletcher told BBC NewsOnline.
In an e-mail to customers, Worldpay said it was doing all it could to combat the attack.
"While such attacks can be anticipated to some extent, it does take time to identify and deal with the exact nature of a particular attack which can also change several times, over several days," the company said.
Owned by the Royal Bank of Scotland, Worldpay has 30,000 clients around the world.
These range from multinational firms such as Vodafone and Sony Music Entertainment to numerous small online retailers.
Worldpay operates in more than 70 countries. It accepts payments on credit cards including Visa, Mastercard and American Express.
Snorestore, a Worldpay customer which supplies specialist earplugs and eye care products, said its orders had dried up almost totally over the weekend.
Director Jessica Gooch said many customers would have been put off by the technical problems they had encountered and gone elsewhere.
"It could not have happened at a worse time for us because people tend to access the site more often at the weekend," she told BBC NewsOnline.
Gooch said she believes the firm lost about 100 orders last November when Worldpay was hit by a similar attack.
"What is most frustrating is that we were assured that they would put systems in place to insure it would not happen again and less than a year later it has happened again," she said.
More attacks on the way?
Malcolm Seagrave, an internet security expert with telecoms firm Energis, said any business connected to the internet was vulnerable to this form of hacking.
"The busy Christmas retail period means these attacks will become more and more prevalent and high profile over the coming months."
Worldpay could be not be blamed for falling victim to the attack, Mr Seagrave added.
"Security cannot be implemented properly at the customer level for this type of attack," he said.
"Customers must demand this type of their protection from their internet service providers."
Have you been affected by Worldpay's problems? If so tell us your experiences.
We launched a new Internet service www.L-Mail.com last week. It lets people send real letters online. We use Worldpay as the payment engine.
This outage could not have come at a worse time for us. We spent all last week promoting the service and the Worldpay outage over the weekend has made our service unusable by the many potential clients who visited our site. We only hope they will return.
Thankfully, the payment system does appear to be returning to normal, although we've been unable to log into the Worldpay administration system for some time.
Peter Harris, Northampton, UK
I tried to order a blind online on Sunday. When it got to the payment part the WorldPay page would not appear. So I went elsewhere and bought from another online retailer using a different payment system. The original company, based in Glasgow, lost my custom because of it.
Daniel Barron, St Albans
It is unfair to criticse WorldPay - it is the hackers who should be blamed.
Richard, Cambridge, UK
I run my ecommerce business through worldpay and although the service is running very slow it is still operating. We have lost 50% of our business since the attack.
I am in the process of moving my company to a better merchant account - so I'm glad they are getting hit.
We use WorldPay to handle all of our online orders as we sell web hosting and domain names it is vital that prospective customers can see that our operations are smooth and without fault.
We have lost hundreds of pounds in lost orders as a result of WorldPay's inability to stave off attacks.
Matthew Prust, Brighton, UK
Orders have stopped over the weekend, despite having more than the usual number of visitors to the site - the build up to Christmas is just begining, so this couldn't have happened at a worse time. We were also adversely affected last year when this happened, for over a week.
Nigel, Cramlington Northumberland
We rely on Worldpay for our online card processing and have not had a successful order since Sunday.
Luckily we are able to view our customers shopping baskets and have been contacting them directly to complete orders via the real shop.
I really had hoped that WorldPay had learnt its lessons last year, this reflects badly on both of us.
Robert Johnston - The Gentleman's Shop, Hungerford, Berkshire
Even though we have a notice on our shopping cart requesting customers to phone in their order if it can't be processed by WorldPay, generally they don't bother - they give up. This weekend our sales are down by about 75% which is disasterous to a small family home based business as our is.
Malcolm Goode, Wallasey, United Kingdom
Our business (Handy Hippo) sells card making and craft supplies on-line and we have seen a significant reduction in business over the weekend. Orders are still getting though, but many orders are failing to complete and customers have been getting in touch for help.
Since many of our customers are trying on-line shopping for the first time, a bad experience like this may put them off for years to come.
Richard Heywood, Farnham Common, Bucks
We used to use WorldPay as the payment processor for our online store, www.picnicshop.co.uk. We moved to another payment processor partly because of the Denial of Service problems WorldPay had last year.
It is difficult for companies, even if they are as large as WorldPay, to prevent these things happening but we cannot have days and days of interupted service again.
Mark, St Albans
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