Cyber criminals are spreading ransomware to Windows 10 users in fake emails offering system updates, according to internet security firm Bitdefender.

Ransomware known as CTB Locker tempts users with the promise of a Windows 10 installation kit. Once the file is downloaded and installed it presents a message saying that crucial system files are now locked. The hackers then demand payment of £360 in bitcoins within 96 hours.

The malicious email has the subject line ‘Windows 10 Free Update' and is reportedly sent from a valid address that displays as ', said Bitdefender.

Bogdan Botezatu, senior threat analyst at Bitdefender, explained that this version of CTB Locker creates the "perfect context" for cyber criminals.

"Millions of people are expected to upgrade to Windows 10, so we might witness a substantial number of PC users falling victim to such scams," he said.

CTB Locker, the successor to Cryptolocker, is a file-encryptor malware said to have originated in spam centres in the US, India and France.

The best line of defence is to back up files regularly, Catalin Cosoi, chief security strategist at Bitdefender, told V3.

"To block ransomware, users need to install an anti-malware solution with anti-exploit, anti-malware and anti-spam modules and keep all of the installed software constantly updated," he said.

"Old or unpatched software is the primary vector used by hackers to infect computers. Good internet practices are not be neglected.

"Avoiding questionable websites, links or email attachments from uncertain sources is the most simple measure any user can take against infections."

Cosoi told V3 that the security of Windows 10 "shows signs of continuous improvement", but that "as long as it is able to run legacy apps it will also run the existing malware targeting Windows platforms".