The messages, which vary by ISP, appear during browser sessions when a user tries to access any website. BT, Sky,TalkTalk and Virgin Media are required to ask all their customers if they want web filters turned on or off, with the government saying it wants to create a "family friendly" Internet free from pornography, gambling, extreme violence and other content inappropriate for children. But the measures being taken by ISPs have been described as "completely unnecessary" and "heavy handed" by Internet rights groups.
The hijacking works by intercepting requests for unencrypted websites and rerouting a user to a different page. ISPs are using the technique to communicate with all undecided customers. Attempting to visit WIRED.co.uk, for example, could result in a user being redirected to a page asking them about web filtering. ISPs cannot intercept requests for encrypted websites in the same way."
Link to Original Source