UK customers of the US bank, MBNA, are being targeted by an e-mail scam.
Some customers are receiving spoof e-mails
Some of its customers have been sent fraudulent e-mails asking them for their online bank details.
MBNA is just the latest of the High Street banks affected by the scam, known as phishing.
"Customers who may suspect they are the recipients of this spoof should call MBNA immediately," said Martino Corbelli, of e-mail filtering company SurfControl, which uncovered the fraud.
It said it had seen a large increase in a spoofing spam in the past twenty-four hours posing as the MBNA Europe bank.
UK customers of MBNA have been sent an e-mail with the bank's branding.
It comes with a variety of subject lines such as "MBNA's OfficiaI Notice," "Attention all MBNA users" and "0fficial Notice for all users of MBNA."
The message says the bank is putting in a new security system to "help you avoid frequently fraud transactions and to keep your investments in safety".
It then advices people to reactivate their account by clicking on a link in the message.
The link leads to a professional-looking website with the MBNA colours, but with a different web address to the one used by the bank.
"Any customer who tries to login through this fake page is very likely to have their personal bank information or identity stolen and relayed directly to the spammers," said Mr Corbelli.
An MBNA spokeswoman said the bank's security had not been breached but customers were being found by the scammers randomly generating likely e-mail addresses.
"We are in the process of contacting our online banking customers to advise and educate them about this matter," said the spokeswoman.
She said MBNA would never ask for personal information by e-mail and added: "If customers respond to the fraudulent enquiry by providing personal information they should contact MBNA immediately."
She said many other UK and US banks had been bit by phishing scams over the past 12 months.
Phishing has been used by fraudsters and organised crime to get customer bank details.
Some leading financial institutions have had their names used in "spoof" e-mails.
In December, NatWest temporarily suspended its internet banking facility after some of its customers were sent fraudulent e-mails asking them to divulge their account details.
In October, Nationwide and NatWest were targeted by a similar hoax as was the Halifax, while in September fraudsters tried to trick customers of Lloyds TSB and Barclays.