Chris Wraight, technology consultant at anti-virus firm Sophos, said Klez victims had 'no excuses" for not updating their anti-virus software'.

'In its various guises, Klez has now been hanging around the chart for thirteen months, making it the most persistent worm ever. People infected by Klez couldn't have updated their virus protection in quite some time,' he said.

Klez was most frequently occurring infection in February, accounting for almost 14% of the month's total incidents, according to figures from Sophos.

The research also found that 541 previously unknown viruses, worms and Trojan horses emerged during last month.

Wraight expressed surprise that the much-hyped Lovegate worm was relatively insignificant, rating only ninth in the top ten.

'The new Lovegate worm may have been the most talked about virus in February,but it certainly wasn't the most prolific, reaching only number nine in thechart,' he said.

Virus hoaxes continue to be a nuisance to users with 'JDBGMR' the most common.

'This needn't be the case. People receiving this message should simply delete it and not be tempted to forward it to all their contacts 'just in case' it is a real virus warning,' advised Wraight.

For February 2003, the virus chart is as follows, with the most frequentlyoccurring virus at number one:

1. W32/Klez-H (Klez variant) 13.7% 13th MONTH WITH KLEZ IN TOP TEN
2. W32/Sobig-A (Sobig worm) 7.7%
3. W32/Avril-B (Avril variant) 6.0%
4. W32/Yaha-E (Yaha variant) 4.6%
5. W32/Bugbear-A (Bugbear worm) 4.3%
6. W32/Avril-A (Avril worm) 3.1%
7=. W32/Klez-E (Klez variant) 2.4%
7=. W32/Yaha-K (Yaha variant) 2.4%
9=. W32/Lovgate-B (Lovgate variant) 2.1% NEW ENTRY
9=. W95/Spaces (Spaces virus) 2.1%

Others 51.6%