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The precise number of websites blocked in the United Kingdom is unknown. Blocking techniques vary from one Internet service provider (ISP) to another with some sites or specific URLs blocked by some ISPs and not others. Websites and services are blocked using a combination of data feeds from private content-control technology companies, government agencies, NGOs, court orders in conjunction with the service administrators who may or may not have the power to unblock, additionally block, appeal or recategorise blocked content.