Royal Bank of Scotland (RBS) and Natwest customers were unable to access online or telephone banking over the weekend after maintenance issues.
A glitch in a regular maintenance update on Saturday took the banking services offline over the weekend. ATM machines and credit card payments were unaffected.
An RBS spokesman confirmed all customers with an online bank account were affected, but was unable to provide details of the glitch. All services are now "running largely as normal", he said.
"The issues emerged as a result of major system changes that took place overnight on Friday," said the spokesman in a statement.
He said customers who suffered bank charges as a result of the problems will be refunded. The bank said it is possible some direct debit payments bounced and customers incurred charges.
The RBS and Natwest online banking services downtime follows problems experienced by HSBC and Barclays customers last week.
HSBC customers across the world had their cards rejected as the bank suffered a major technical problem. Customers were unable to use ATMs, online banking services or make credit card payments.
HSBC said the problems were caused by a mainframe outage. Services are now back to normal.
Barclays customers were unable to log on to online bank accounts last week and encountered extended response times on its internet banking services for two days.