Although security experts have issued a red alert over the Bagle.BC worm, the most prevalent and damaging viruses in October continued to be Netsky-P and Zafi-B, viruses which have topped the chart since June.

"With only one new entry, at the bottom of this month's chart, the dominance of Netsky-P and Zafi-B seems unshakable," said Carole Theriault, security consultant at Sophos.

"These viruses are so prevalent and well established that it is very difficult for newer viruses to compete. Thousands of copies are circulating all over the world.

"It may be that unprotected PCs in areas where computer security is not a top priority are responsible for keeping the menace alive."

Sophos' figures show that over one in 22 emails circulating during October were viral. When this figure is compared to those of the past few months, a noticeable reduction in viral email traffic can be seen. In July, over nine per cent of emails were viral, the company said.

Meanwhile, the trial of Sven Jaschan, the self-confessed author of the Netsky worms, has been postponed until 2005.

"The German court is said to have delayed the case because it has proven impossible to agree an earlier date with all parties involved," said Theriault. "Users affected by Jaschan's worms will no doubt be waiting to see the outcome."