Levels of worldwide spam dropped instantly by up to 75 per cent after hosting company McColo Corp was shut down by two major internet service providers on Tuesday.

Many security experts believe McColo was the source of the majority of worldwide junk email, many of which carrying such items as malware and viruses which caused numerous security headaches for IT support experts.

Within seconds of the networks being closed down in northern California, spam levels had dropped significantly, Jason Steer of Cisco/Ironport told SC Magazine.

He commented on the business that ran the botnet, which was well known for a long time due to its ability to blame "customers" for the problems caused by spam to police as they had been able to "set up a legitimate business at the front end".

Botnets are computer networks controlled by illegally-installed software run from a separate computer as opposed to the owners themselves, according to Tech-FAQ.

More than half of small businesses (53 per cent) believe that the most important benefit of outsourcing is guaranteed response times for IT support. London-based Connect conducted the research in 2007.