The effort is believed to be the biggest scheme of its kind by an internet service provider (ISP) to bar its customers from child porn sites. It comes as law-enforcement officials around the globe step up pleas to the industry for help.
BT announced on Monday it would be installing an elaborate software filter to block access to a list of suspected child porn sites. The "block list" will be compiled by watchdog group Internet Watch Foundation and vetted by the Home Office.
The scheme, known as Cleanfeed, began as a trial on Monday and will be made available to BT's base of one million retail customers in the coming weeks, the company said.
The availability of child porn on the internet has become one of the biggest areas of cybercrime. Police forces around the world have been rounding up thousands of Internet users accused of accessing such sites.
The ongoing Operation Ore has identified 7200 Britons alone who supplied credit card details to access a now defunct child porn site operated out of the United States.
The availability of child porn is one
of the biggest areas of cybercrime
In the UK, the Child Protection Act of 1978 makes it illegal to view images of child pornography. "In the UK, because it is illegal to view these images, we can stop an illegal activity," Pierre Danon, head of BT retail services, told Reuters.
He cautioned the same blocking scheme would not be extended to websites inciting hatred or espousing terrorism or those that traffic in copyrighted materials, citing the lack of clearly worded laws outlawing such content.
"We do not intend to, nor can we, extend this to other areas," Danon said.
Cleanfeed works by intercepting an internet user's request to access one of many sites on the block list.
The request will go through two filters, and, if it is determined it is attempting to access a known child porn site, the user will get a "Web site not found" error message.
BT said that while the user's request will be blocked, their personal details would not be collected.
The company, Britain's largest wholesale and retail provider of high-speed broadband access over ordinary phone lines, said the service would affect retail subscribers only including customers of BT Yahoo, its joint venture with web firm Yahoo Inc.