The National Crime Agency is warning of a massive spamming event targeting UK computer users by email.
The agency has said people are "receiving emails that appear to be from banks and other financial institutions" - in large numbers.
"The emails may be sent out to tens of millions of UK customers, but appear to be targeting small and medium businesses in particular. This spamming event is assessed as a significant risk," it said.
"The emails carry an attachment that appears to be correspondence linked to the email message (for example, a voicemail, fax, details of a suspicious transaction or invoices for payment). This file is in fact a malware that can install Cryptolocker – which is a piece of ransomware"
Lee Miles, Deputy Head of the group's cyber crime unit said "The NCA are actively pursuing organised crime groups committing this type of crime.
"We are working in cooperation with industry and international partners to identify and bring to justice those responsible and reduce the risk to the public."