The censorship scheme, dubbed Cleanfeed, has been backed by the Home Office.
BTInternet and BTYahoo users will be sent to an error message if they try to access the illegal websites, the company said.
BT will use a black list, provided by the Internet Watch Foundation, to block the sites. The list includes sites that are illegal to view according to the Child Protection Act.
Other ISPs are talking to the firm about doing the same.