Antivirus firm F-Secure has apologised for sending the Netsky.B virus to several thousand of its UK customers and partners via a mailing list.
The email apology said: "Because of a human error, you may have received an email infected with the Netsky.B virus that was relayed through our external email list server and was resent to our UK mailing list.
"The virus did not originate from our network - it was sent by an unknown party to the list address. If you had up-to-date antivirus installed, the virus has been stopped automatically already and no further steps are necessary on your part."
Mikko Hypponen, F-Secure's director of antivirus research, said the mailing list was outside of the firm's normal email scanning.
"There was no need for us to accept external emails being sent to the list. We have rectified this, and contacted the people on the list to warn them," he told vnunet.com.
He said steps had been taken to ensure this could not happen again.
The Netsky.B worm spreads itself in emails inside a .zip archive or as an executable attachment. It also copies itself to shared folders of all available drives. This allows the worm to spread through peer-to-peer and local networks.
One recipient, who wished to remain anonymous, told vnunet.com: "It's not very good is it, coming from an antivirus firm? You'd expect better."