Customers of Nationwide have been unable to access the building society's website - the gateway to its online banking operation.
Customers were met with a message on the website's front page that said the "server is too busy".
The problem, which started at about 1115 GMT, meant customers had to go directly to the online banking page to make transactions.
A link to the online banking page was added to Nationwide's homepage later.
Nationwide said the glitch was fixed, and the website was operating normally again, by 1345GMT.
The UK's largest building society - like many financial institutions - has suffered problems before with its computer system.
In June 2009, the online banking facility was inaccessible for most of one day.
The previous December, a "processing problem" meant some accounts were not updated with automatic end-of month credit and debit payments, such as salary payments.
Other banks and building societies have seen temporary faults with online banking since millions of customers started using it day-to-day. Other automated problems have left some banks printing other customers' details on statements.