Natwest has been targeted in a cyber attack which prevented customers from accessing its website.
Parent company RBS Group said that the bank was subject to a distributed denial of service (DDOS) attack on Friday, with many customers unable to access online banking for an hour.
"Due to a surge in internet traffic deliberately directed at the NatWest website, customers experienced difficulties accessing some of our customer web sites today," the bank said in a statement.
"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.
"The disruption began at 11:36am and ended at 12:30pm. We apologise for the inconvenience caused."
RBS and Ulster Bank customers were also briefly impacted by the cyber attack, with all websites now returning to normal service.
It is not believed that customer data was compromised in the attack, with the attack focused on disruption rather than an attempt to retrieve customer details. RBS is now working to identify the origin of the attacks.
The bank said that the problems are not related to unspecified IT problems earlier this week, which resulted in problems with online and mobile banking, ATMs and debit card payments.
On Wednesday RBS CEO Ross McEwan apologised for the outage, stating that the bank had experienced historic underinvestment in its technology systems.
"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."