Security administrators are being warned of a huge surge in incidents of the Bredolab Trojan, which could allow hackers to gain complete remote control of an organisation's PCs.

Researchers at hosted security firm MessageLabs said that the Trojan, which is being sent out by the Cutwail botnet, has reached its highest ever levels, and now accounts for 3.5 per cent of all spam and 5.6 per cent of all malware intercepted each day.

MessageLabs, which is now part of Symantec, said that so far in October around 3.6 billion Bredolab malware emails are likely to be in circulation globally each day.

The Trojan is likely to appear in a zip file attachment to an unsolicited email with a subject line referring to postal tracking numbers, said the firm. The email prompts the recipient to open and run the attachment, which automatically installs the Trojan.

"By nature, once this Trojan is on a system, it is unlikely to be detected and will allow the controller to do whatever they wish with the infected machine, such as installing other malware and spyware," said MessageLabs senior analyst Paul Wood.