New York ? A computer virus that circulated across the Internet this week, hard on the heels of another nasty online infection, is the fastest e-mail outbreak ever, an anti-virus company said.

MessageLabs, which scans e-mail for viruses, said that within 24 hours it had scanned more than 1 million copies of the “F” variant of the “Sobig” virus, which was blamed for computer disruptions at businesses, colleges and other institutions worldwide.

The previous record was “Klez,” with about 250,000 copies spotted during its first 24 hours earlier this year, MessageLabs chief technology officer Mark Sunner said Thursday.

There have been faster outbreaks on the Internet, but those circulated through networking functions built into Windows operating systems.

The “Slammer” worm struck more than 75,000 computers in just 10 minutes in January, with the number of infected computers doubling every 8.5 seconds, according to researchers at the University of California and other institutions. It went on to infect hundreds of thousands more.

E-mail viruses like “Sobig” can hit the same computer multiple times, so the number of infections is not directly comparable.