ALBANY  Verizon, Sprint and Time Warner Cable have agreed to purge their servers of Internet bulletin boards and Web sites nationwide that disseminate child pornography.

The move is part of a groundbreaking agreement with the New York attorney general, Andrew M. Cuomo, that will be formally announced on Tuesday as a significant step by leading companies to curtail access to child pornography. Many in the industry have previously resisted similar efforts, saying they could not be responsible for content online, given the decentralized and largely unmonitored nature of the Internet.

The agreements will affect customers not just in New York but throughout the country. Verizon and Time Warner Cable are two of the nation’s five largest service providers, with roughly 16 million customers between them.

Negotiations are continuing with other service providers, Mr. Cuomo said.

The companies have agreed to shut down access to newsgroups that traffic in pornographic images of children on one of the oldest outposts of the Internet, known as Usenet. Usenet began nearly 30 years ago and was one of the earliest ways to swap information online, but as the World Wide Web blossomed, Usenet was largely supplanted by it, becoming a favored back alley for those who traffic in illicit material.