The worm, dubbed Mimail, first struck on Friday in the US but it has enjoyed a resurgence with people returning to work after the weekend.

Anti-virus firm Sophos said it had noticed a big increase in infections, suggesting that employees had gone to work and opened the e-mail worm, causing it to spread to all their contacts.

"The Mimail worm is getting a second lease of life as UK businesses log on to start a new working week," said Graham Cluley, Sophos senior technology consultant.

"While US firms have been patching their systems against this threat, their UK counterparts have been enjoying a sunny weekend, blissfully unaware that a virus is sitting on their e-mail system just waiting to be unleashed."

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