The NatWest personal banking website has been hit by a cyber attack in the wake of its IT woes earlier this week, Sky News has confirmed.
Some customers trying to log on to the website found it impossible to enter the site.
NatWest, which is owned by the Royal Bank of Scotland Group, said there had been a deliberate swamping of its site.
An RBS spokesperson told Sky News: "Due to a surge in internet traffic deliberately directed at the NatWest website, customers experienced difficulties accessing some of our customer web sites today.
"This deliberate surge of traffic is commonly known as a distributed denial of service (DDOS) attack.
"We have taken the appropriate action to restore the affected web sites. At no time was there any risk to customers. We apologise for the inconvenience caused."
The bank stressed that the problems on Thursday night and Friday were not connected with its banking blackout which began on Cyber Monday - the biggest online retail day of the year - and stretched into Tuesday.
Some technical problems continued until Wednesday and thousands of customers who were unable to use the banks' websites or card services vented their fury online.
The group chief executive Ross McEwan described the earlier glitch as "unacceptable" and added: "For decades, RBS failed to invest properly in its systems.
"We need to put our customers' needs at the centre of all we do. It will take time, but we are investing heavily in building IT systems our customers can rely on.
"I'm sorry for the inconvenience we caused our customers. We know we have to do better.
"I will be outlining plans in the New Year for making RBS the bank that our customers and the UK need it to be.
"This will include an outline of where we intend to invest for the future."
As well as this week's problems, a glitch in May left RBS and NatWest customers using mobile apps unable to access their accounts online.
That followed a major fiasco in June last year which saw payments go awry, wages appear to go missing and home purchases and holidays interrupted - and cost the group £175m in compensation.
:: Watch Sky News live on television, on Sky channel 501, Virgin Media channel 602 and Freeview channel 82