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A spokesman for the National Crime Agency has said that the best advice is not to open any attachments in emails that appear to be from your bank.
If you are in any doubt at all, call your bank to ask whether they sent the email.
Cyber crime officers have said that the malicious emails often appear to be genuine communications from a bank.
They come with attachments that appear to be files such as a voicemail, fax, an invoice or details of a suspicious transaction.
If you receive such an email:
- Do not to click on any attachments
- Update your antivirus software and operating systems
- Back up important files
If your computer is infected with malicious software:
- Disconnect it from the network
- Call a professional to clear the machine
- Report it to www.actionfraud.police.uk
Lee Miles of the National Cyber Crime Unit (NCCU) has said his agency is working with international Partners to find the source of the malicious emails:
Tens of millions of UK email users may be having their financial details stolen in a "mass spamming event," according to experts in cyber crime.
The National Crime Agency warned that small and medium businesses are the top targets of the messages that appear to be legitimate emails from banks.
The emails come with attachments that appear to be files containing details of a suspicious transaction, but in fact harbour malicious software.
The screen will then display a countdown timer that demands the payment of 2 Bitcoins (around £536) - a form of currency used for Internet transactions - in ransom for the decryption key.
The National Cyber Crime Unit advises that anyone whose computer is infected should report it to www.actionfraud.police.uk.