Barclays Bank has warned of a sophisticated new phishing e-mail which takes users to a fake Barclays site requesting security details, as well as opening the bank's real site in a separate browser.
In issuing the warning, Barclays reiterated that it never requests security details by e-mail.
The security threat comes just days after Barclays recommended that its online banking customers buy discounted F.Secure internet security software to protect their online activity.
The F.Secure software is offered at a discount to customers via the Barclays website, and includes anti-virus and anti-spyware protection.
The F.Secure software will be offered to all Barclays' online account holders. The bank said the move was part of its drive to reassure customers that online banking is safe and convenient.
David Mitchell, senior operations manager at Barclays, said, "When it comes to online banking, acting responsibly means not just ensuring that our own processes are completely secure, but also that the customer's computer is protected, so that the whole transaction is totally safe."
Nationwide is telling users of its online banking service that any e-mail sent from the building society will include a bitmap image featuring the customer's postcode.