The number of junk e-mails being sent to computer users around the world has fallen by nearly two thirds after the plug was pulled on an American computer company thought to be linked to some of the biggest junk-mail gangs.

Experts said that spamming had fallen dramatically this week, after McColo Corp, a Californian company that has been accused of providing the gateway for much of the world’s junk e-mail, was shut down.

Spam or junk e-mails are unsolicited messages sent in bulk. Most are advertisements for dubious products and fake get-rich-quick schemes.

One recent study showed that even though only one in every 12.5 million junk e-mails receives a reply, spammers still turn a healthy profit. This is because more than 100 billion are sent a day, the vast majority from fewer than 200 spammers.

In what is being seen as a significant victory against junk e-mailers, security researchers accumulated evidence of alleged spamming activities at McColo, and persuaded its internet providers to get the web-hosting service taken down last week.

It remains unclear whether any crime has been committed by McColo. American law enforcement agencies, including the FBI, would not comment. Representatives from the company could not be contacted, and its website is no longer working.

It is notoriously difficult to target and shut down sources of spam. Investigators have to prove that company officials know that crimes are being committed through their servers. Web-hosting companies often argue that they do not monitor how customers use their services.

Security analysts told The Times that as spam messages affected all e-mail addresses around the world equally, British customers would also experience a 65 per cent decrease in junk e-mails. Other internet security firms suggested the fall could be as high as 75 per cent.

In the case of McColo, it appears that spam senders used the company’s servers to send commands to huge numbers of computers that they had in effect hijacked.

Spammers use networks of these compromised computers – known as “botnets” – to amass enough computing power to send millions of messages a day. The vast majority of owners of those machines do not know their computers are secretly being used for this purpose. But spammers need a way to communicate with these computers plus a web-hosting company willing to look the other way.

The researchers who led the investigation into McColo’s activities allege that some of the world’s biggest botnets operated through the company’s servers.

As of Tuesday last week, about 153 billion e-mail messages were being sent every day. Within two days of McColo being closed this had dropped to 64 billion, according to IronPort, an internet security company.

The respite from an infested inbox is likely to be brief, however. Experts said that spammers were already scrambling to connect to millions of PCs infested with viruses which were once used to send spam.

“Spammers will send out ‘malware’ on to your computer through spam or infected websites,” said Amanda Grady from Symantec, the internet security firm.

“Once this malware is on a person’s machine, so long as they are connected to the internet and completely unknown to them, their machine could be sending out spam.”

“This is a temporary reprieve,” said Nilesh Bhandari from IronPort. “We should enjoy it while we can.”

Comedy sketch that became a worldwide problem

— The term “spam” is thought to have been inspired by a Monty Python sketch, in which a restaurant sells only Spam dishes. A group of Vikings start singing: “Spam, lovely Spam, wonderful Spam”

— Internet chatrooms started to use spam in a number of contexts until it became best known as a reference to unsolicited bulk e-mail

— The first known spam message was sent 30 years ago when a marketing representative at a computer firm sent an e-mail to every American West Coast user on the Arpanet, considered the prototype of the internet Much spam is sent from home PCs that have been taken over by viruses, and turned into “botnets”, which automatically spew out millions of messages

— More than 90 per cent of e-mails are spam, according to one estimate

— The US is the main generator of spam, with South Korea the second-largest, and China third. Britain is ranked tenth

— Thirty-one per cent of all spam is about “adult-related matters” such as pornography or erectile dysfunction

Sources: BBC; moneysupermarket.com, Spamlaws.com