Three of the biggest US internet service providers have agreed to block access to bulletin boards and websites that carry images of child sex abuse. The firms - Verizon, Sprint and Time Warner Cable - will also provide more than $1m (£500,000) to fund efforts to remove child sex sites.
The agreement was brokered by New York Attorney General Andrew Cuomo.
Previous attempts to crack down on child pornography have been resisted by the industry.
Internet companies had previously argued that they could not be held responsible for how individuals communicate with other individuals online.
Law enforcement agencies therefore concentrated on targeting the producers of images of child sex abuse rather than the ISPs.
'Graphic'
Mr Cuomo's initiative represents a new approach.
"You can't help but look at this material and not be disturbed," the New York Times quoted Mr Cuomo as saying.
"To say 'graphic' and 'egregious' doesn't capture it," he added.
Agents from Mr Cuomo's office conducted an eight-month investigation into the ISPs before the agreement was reached.
They posed as ISP subscribers and complained to the companies that they were allowing images of child sex abuse to proliferate in spite of customer service agreements pledging to discourage such activity.
Mr Cuomo's office threatened the ISPs with charges of fraud and deceptive business practices.
In an attempt to avoid the charges, the firms agreed to Mr Cuomo's terms, including the agreement to block access to child sex sites.
The agreement will affect customers across the US.
Negotiations with other service providers to broker similar agreements are continuing, according to Mr Cuomo.
BBC NEWS | World | Americas | US net firms to block child porn
US net firms to block child porn
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