An e-mail scam is targeting Barclays banking customers to trick them into revealing their confidential details.
The e-mail sent to Barclays clients under the guise of initiating a technical update of their account, takes the customer to a page with a Barclays logo.
It then asks the customer to fill in such data as membership number and password.
The details could be used to withdraw funds and transfer them to various accounts.
Barclays warned customers about revealing their details and said genuine communications from the bank never ask for complete details such a full password or PIN numbers.
Out of 3.5 million customers who do their banking with Barclays online, 400 have so far contacted the bank about the e-mail and a handful have fallen for the scam.
Barclays said its systems "had not been compromised in any way and the bank is taking appropriate action" against the fraud and working with the police.
"Any losses will be covered by Barclays whatever the amount," the bank said.
The e-mail appears to have come from Baltimore in the United States but the company sending it may not realise it is hosting the scam.
The occurrence of such scams has risen from eight last year to 40 this year already, the National Hi-tech Crime Unit says.
Barclays have set up a helpline for customers who think they may have been targeted by the fraudsters. The number to call is 0845 600 2323. Local rates apply.