Yet another version of the Bagle virus has been spotted in the wild, bringing the number of variants discovered today to four and prompting security experts to warn that this could mark the start of a serious wave of virus attacks.

Grisoft, makers of the popular AVG Anti-Virus products, confirmed that its labs had detected Bagle.BM, joining other variants BB, BD and BE reported earlier today.

Bagle.BM is a 34KB Windows executable file. It is attached to messages which come with an empty subject line. The body contains the words 'new price' or just 'price'.

When a user opens the attachment it activates the worm, which copies itself to the Windows system directory and registers this file in the system registry.

Bagle.BM also terminates processes designed to protect the machine and the local network. This leaves the infected PC vulnerable to further attacks by malicious code, Grisoft warned.

"The Bagle BM variant is one of several new strains seen by our labs this morning," said Michael Foreman, partner at AVG UK.

"With several months since the last major outbreak, it looks like we may be entering a new period of increased virus activity.

"We are seeing an increase in the amount of malware sent by spammers. It has become the preferred method for expanding their network of infected machines used to deliver spam messages."