The Bank of England has warned people to ignore a hoax e-mail which has gone out in its name urging people to install security software.
The Bank is the latest victim of "phishing"
The Bank warned that files attached to the message should not be downloaded in case they were infected with viruses.
Up to 100,000 people have replied to a forged e-mail address - the first time the Bank has been targeted by a scam.
Experts say the ploy, called "phishing", is used by fraudsters and organised crime to get customer bank details.
The e-mail purported to come from an administrator at the Bank and claimed that downloading the software would help people combat credit card fraud.
The Bank immediately launched an investigation but said it was unclear where the e-mail had originated from or who had sent it.
A spokeswoman said the Bank's systems had not been affected.
Technicians are now checking the e-mail from the address firstname.lastname@example.org and the attached software file to see if it contained a virus.
A spokeswoman for the Bank said there may be many more than 100,000 such emails in circulation as the figure only reflects "bounce backs" from people who are not in their offices.
She added: "We have already taken some calls from members of the public and some banks which have been sent this e-mail. Our advice to people is to delete
this e-mail immediately."
Some leading financial institutions have had their names used in "spoof" e-mails.
This month, NatWest temporarily suspended its internet banking facility after some of its customers were sent fraudulent e-mails asking them to divulge their account details.
The con uses e-mails asking customers to verify their details and handover pin numbers and passwords through a replica website to the one targeted, such as a bank.
The messages are sent randomly to consumers, in the hope that someone will divulge their banking details.
In October, Nationwide and NatWest was targeted by a similar hoax as was the Halifax, while in September fraudsters tried to trick customers of Lloyds TSB and Barclays.