Clydesdale and Yorkshire Bank has apologised to customers over IT problems at the weekend which saw its website crash.
Bank customers were unable to access the website for most of Sunday as the domain name was not routing properly.
The company says the problems have been resolved barring some residual issues around internet service providers.
The lender says it is looking into what caused the problem but did not deny reports it was caused by not renewing the domain registration.
A Clydesdale and Yorkshire spokesman says: “We have had a problem with our internet banking service, which has been fixed, the majority of customers should now be able to log on as normal.
“In some specific cases, customers may still be unable to use the service but this will be resolved as systems update over the next 24 hours. We are sorry for any inconvenience to our customers.”
Last year the bank suffered a number of issues stopping customers using its debit and credit cards while in 2010 it write to 18,000 customers about mortgage underpayments.
In its half-year results to the end of March, published by parent National Australia Bank in May, the bank set aside £38m to deal with “customer redress” including £15m for interest rate swap misselling.