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Levels of spam have fallen by almost 50% since August 2010, suggest figures.

Figures show that the amount of junk e-mail messages flowing around the net has dropped 47% in three months.

Kaspersky Labs noted a similar fall from July to September, when spam levels fell to 81.1% of all e-mails

The decline was put down to the arrests of those behind spam-sending botnets, and intelligence work that saw other spamming systems shut down.

In the last few months security firms have scored several notable successes against gangs that own and operate botnets - collections of hijacked home computers.

The vast majority of spam or junk mail is routed through these hijacked machines.

One of the biggest successes was against the Pushdo or Cutwail botnet, which had been in operation since 2007 and was thought to be sending about 10% of global spam.

An international operation co-ordinated by the security firm LastLine managed to get 20 of the 30 servers controlled by the group shut down. The servers were turned off with the help of the internet service providers unwittingly found to be hosting them.

As a result, many of the "drone" PCs in the huge botnet used to send e-mail were cut off and no longer relayed the junk messages.

Bredolab was another big botnet hit in October thanks to work by the hi-tech division of the national crime squad in the Netherlands. The arrest of an Armenian man thought to be the botnet's controller led to the closure of the 143 servers linked to Bredolab.

At its height Bredolab was thought to involve up to 30 million computers around the world and be capable of sending 3.6 billion e-mails every day.

Police forces also took action against many of the people involved in the Zeus botnet.

Around the world about 100 people were arrested and many of the command and control machines overseeing the network were turned off.

Spammers were also hit by the September closure of the Spamit partner program. It paid spam senders to promote its Canadian Pharmacy network of sites peddling fake pills.

Action against botnets and the closure of Spamit led spam volumes to drop to 86.8% of all e-mail, the lowest percentage since September 2009, said Symantec.

However, Kaspersky analyst Darya Gudkova warned that there was bad news mixed in with the good.

"Spam is becoming a greater threat as it now frequently contains a variety of malicious attachments and links to infected websites," she said.