"The closure of a web hosting firm that is believed to have had spam gangs as clients has led to a drastic reduction in junk mail.

Two US internet service providers have pulled the plug on the firm McColo following an investigation by the Washington Post newspaper.

Anti-spam firm Ironport has seen junk mail levels drop by 70% since McColo was taken offline on 11 November. But, it warned, it will be a temporary respite from the menace of spam.

"It is an unprecedented drop but will be a temporary outage as the networks move from North America to places where there is less scrutiny," said Jason Steer, a spokesman for Ironport.

The Washington Post has been gathering data on McColo for the past four months and passed the information to its internet service providers, Global Crossing and Hurricane Electric.

Both decided to pull the plug on the firm on Tuesday.

It is believed that it hosted gangs running botnets - networks of computers that have been taken over by criminals to send malicious software and spam. "