Police warn of viral scams disguised as messages from banks, followed by demand for Bitcoin payment to unlock devices

British computer users are being warned of a mass spamming attack that aims to encrypt their computer and extort a £540 ransom – in Bitcoins - to unscramble it.

The UK's National Crime Agency put out an alert on Friday afternoon warning that emails masquerading as communications from banks and financial institutions are being targeted at small and medium businesses, as well as millions of bank customers.

The emails have an attachment which appears to be a voicemail, fax, invoice or details of a suspicious transaction that is being queried.

But instead it is a malicious program that, if opened, will encrypt the user's hard drive and then begin displaying a countdown timer, while demanding the payment of 2 Bitcoins - an almost untraceable online currency. At present exchange rates, 2 Bitcoins costs about £540 - but the price has jumped in the past few weeks.

The NCA warned that the scam is "a significant risk". Lee Miles, deputy head of the national cybercrime unit, said: "The NCA are actively pursuing organised crime groups committing this type of crime. We are working in co-operation with industry and international partners to identify and bring to justice those responsible and reduce the risk to the public."

While such "ransomware" is not new, the Cryptolocker malware being used against unsuspecting users has become increasingly prevalent. The NCA warned that it doesn't endorse paying the ransom to the criminals involved, and there is no guarantee they would honour the payments.

The cybercrime unit is trying to uncover who is sending the emails. Anyone whose computer is infected should report it to Actionfraud.

Users are advised not to click on any attachments like those described and to be suspicious of any emails that appear to come from a bank.

If a computer is infected it should be disconnected from the network and a professional called in to clear the machine – though the current state of encryption technology means it will almost certainly be impossible to unscramble the encryption, and instead the hard drive will have to be wiped and restored from a backup.