The infamous Klez worm has retained the top spot in security firm Sophos' most frequently occurring viruses table for the 15th month in a row, while 475 new viruses were discovered in April.

Two old favourites also reared their heads again — Lovegate-E and Funlove — spreading concern that individuals and companies are not keeping their antivirus software and firewalls up to date.

"It's no good installing the software if it's not maintained," said a company spokesman.

A new virus Datemake Dialler also made its way into the top 10. The malware, known as a Dialler, is programmed to dial a premium rate telephone line, typically with the intent of gaining access to adult material.

"Businesses should apply strict computing guidelines to avoid getting stung by a huge telephone bill and embarrassed by these seedy programs," said Graham Cluley, senior technology consultant at Sophos.

Top viruses were:

1. W32/Klez-H 12.7% of complaints.
2. W32/Lovgate-E 4.9%
3. W32/Bugbear-A 4.3%
4. W32/Sobig-A 3.3%
5=. W32/ElKern-C 2.9%
5=. W32/Yaha-E 2.9%
5=. W32/Yaha-K 2.9%
8. JS/NoClose 2.1%
9. W32/Flcss 1.9%
10. Dial/Datemake-A 1.6%