Bank of England issues warning over hoax e-mail.
It rushed out an alert after more than 100,000 replies to the e-mail were blocked overnight by its internal computer system, which did not recognise the sender's address.
The e-mail purported to come from an administrator at the Bank and claimed that downloading the software would help prevent credit card fraud.
The Bank of England immediately launched an investigation but said it was unclear where the e-mail had originated from or who had sent it.
Technicians were checking the e-mail from 'email@example.com' and its attached software file - antikeylog2004.exe - to see if it contained a virus that could attack computer networks.
The bogus e-mail stated, 'Dear customer. The security of your personal and account information is extremely important to us.
'By practising good security habits, you can help us ensure that your private information is protected. Please install our special software, that will remove all the keyloggers and backdoors from your computer.'
A BoE spokeswoman said the number of the e-mails in circulation could be much higher than 100,000 as this 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.'
Hoax e-mails have caused considerable disruption to banks this year with many high street branches forced to warn customers against giving out their account information.
Nationwide issued a warning in October after people were sent bogus messages trying to trick them into giving away details such as PIN numbers and passwords.