Four of the largest US internet service providers (ISPs) have filed lawsuits against hundreds of people accused of sending junk email messages, the first broad action under a new US anti-spam law that came into effect on January 1st.

The new law, known as the Can-Spam Act, criminalises particular tactics that spammers use to send junk email, such as using deceptive subject lines or routing email through third-party computers. The law, which also provides for civil remedies, gives the lawsuits "added punch", said Mr Randall Boe, general counsel for Time Warner's America Online unit.

AOL was joined in the suit by Microsoft, Earthlink and Yahoo.

Despite dozens of civil lawsuits filed by ISPs in the past few years, the volume of junk commercial email flooding the internet continues to rise, and is estimated to account for as much as 65 per cent of all email.

Unsolicited commercial email has emerged as a leading source of frustration and concern among ISPs and their customers, with hundreds of millions of junk email messages a day driving up bandwidth costs and clogging internet in-boxes.

Mr Matt Robinson of Yahoo said the ISPs were pursuing injunctions to stop spammers from sending additional emails, as well as unspecified damages.

In an unusual show of unity, the four ISPs said they worked together to track down the defendants and ensure their lawsuits did not overlap.

Mr Ron Wyden, a Democrat senator for Oregon, said: "Today's filing proves that the days of spamming with impunity are finally over, and all those who abuse email and threaten its viability as the internet's most popular and useful application should take notice."

Mr Wyden co-sponsored the Bill with Mr Conrad Burns, a Republican senator from Montana.

Law enforcement officials were noticeably absent, however, as the ISPs announced their civil lawsuits.

Attorneys for the four companies acknowledged they did not work with law-enforcement officials in preparing the lawsuits. But they said that did not rule out criminal charges in the future.

Critics of the new law have argued it does nothing to bolster ISPs' ability to sue spammers and it does not provide for additional resources to enforce criminal provisions of the act.

"It's always a good thing when an ISP sues a spammer, but if anyone thinks this will turn the tide, they are sadly mistaken," said Mr David Kramer, an attorney at Wilson Sonsini, a Silicon Valley law firm.

Mr Kramer said ISPs were under a great deal of political pressure to take action under the new law, which specifies that plaintiffs can seek civil damages of up to $3 million (€2.45 million) per defendant.

Only six of the defendants in the lawsuits were identified by name. The rest were identified by their internet addresses or website URLs.

Once a lawsuit is filed, ISPs can seek subpoenas that would help them identify those they accuse of sending the spam.