Customers have been met with a message on the website's front page that says the "server is too busy".

The problem, which started at about 1115 GMT, means customers must go directly to the online banking page to make transactions.

A link to the online banking page was added to Nationwide's homepage later.

Those who bookmarked the page on their computers can also access it.

Nationwide said staff were working to fix the glitch as quickly as possible.

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.