Netsky-P, the worm written by German teenager Sven Jaschan, continues to be the leading cause of computer infection, according to anti-virus company, Sophos.

The top ten chart of worldwide infection in business computers in October, is also dominated by six variants of the Mytob virus.

Total number of viruses reached a record level of 1,685.

Carole Theriault, a senior security consultant at Sophos, says there has been an increase in the onslaught this month as virus writers step up attempts to extort money from businesses.

'Half of the Mytob variants in the top ten are new entries, whose creators appear to be a gang. By working as a group they have the advantage of being able to issue several different variants in a short space of time,' she said.