Internet users could notice a decrease in junk e-mail after the arrest of a man described as one of the world's "most prolific spammers".

Robert Soloway (27) is accused of using networks of compromised computers, known as bot nets, to send out millions of spam e-mails.

"He's one of the top 10 spammers in the world," said Tim Cranton, senior director of Microsoft's Worldwide Internet Safety Program. "He's a huge problem for our customers. This is a very good day."

IE Internet, an Irish-managed e-mail firm, estimates that more than 60 per cent of e-mails sent to Irish e-mail addresses in April was spam, with the bulk of the unsolicited e-mail coming from the US and Korea.

A US federal grand jury returned a 35-count indictment against Mr Soloway last week, charging him with mail fraud, wire fraud, e-mail fraud, aggravated identity theft and money laundering.

Mr Soloway pleaded not guilty to all charges yesterday after a judge determined that - even with four bank accounts seized by the US government - he was sufficiently well-off to pay for his own lawyer.

Justin Mason, the Irish-based developer of the open-source Spam Assassin software which filters spam, said it was hard to tell how much spam aindividual such as Mr Soloway was responsible for sending.

"It's very hard to tell because they try to hide all signs of their activity," said Mr Mason. "From talking to other anti-spam people today, the consensus is he was one of the big guys and probably still is."

Mr Soloway had been living in an expensive apartment and drove an Mercedes convertible, said prosecutor Kathryn Warma.

Prosecutors are seeking to have him forfeit $773,000 (€574,500) they say he made from his business, Newport Internet Marketing.

Mr Soloway continued his activities even after Microsoft won a $3.5 million civil judgment against him in 2005. If convicted in a federal court, Mr Soloway could face decades in jail. - (Additional reporting, PA)