The incidence of malicious software designed to hold personal data to ransom is up by a third, according to security experts.

Panda Software reported a 30 per cent increase in software demanding payment to release computer files during the second quarter of the year. 

So-called 'ransomware' threatens to delete or encrypt users' documents unless a payment is made.

Troj/Ransom-A, spotted in April this year, threatened to delete one file belonging to the user every 30 minutes until a $10.99 ransom demand was fulfilled. 

To prevent being traced, the blackmailer asked for the money to be paid via Western Union. Once payment had been made, users received the code with which to disable the Trojan and recover the files.

Archiveus, which encrypts files and then password protects the files, emerged a month later. 

Users who tried to access the files were directed to a new file containing instructions on how to recover the data. The password to decrypt the file was later discovered and released by security firm Sophos.

In order to protect against such threats, Panda Software urges users to keep their antivirus software up to date and make regular backups of important files.

Users should be especially wary of unsolicited email and P2P software as these are the principal channels for spreading ransomware.