Four prominent Internet Service Providers in the U.K. will require customers to opt in if they want to view pornography, according to a new report.

According to the Guardian, Prime Minister David Cameron later today will unveil the details of the plan, which will prevent customers of ISPs BT, Sky, TalkTalk, and Virgin to access pornographic Web sites without first opting in.

Although full details on how users will be asked to opt in and whether they'll need to do it once or every time they access a site is unknown at this point, in a follow-up report, Guardian said that there are some major limitations to the initiative.

Speaking to the Guardian, a spokesperson for TalkTalk said that the initiative will only apply to customers who sign new contracts, and they will be given the option to sign a deal with "parental controls" or one without that clause. So, folks who have existing contracts will be grandfathered in.

That aspect of the initiative is important. As the Guardian points out, very few Internet users enter into new contracts every year. According to the publication, just 5 percent of customers change their ISP in a given quarter.

The idea behind the initiative is to shield children from pornography, the Guardian says, and was first proposed by the Mothers' Union, a Christian charity "that seeks to support families worldwide." The group has also petitioned Cameron to force content providers to put age restrictions on music videos and require retailers to sell clothes for children that are considered "age-appropriate," the Guardian said.

Aside from its opt-in program, the U.K. will also allow parents or guardians to use a site called Parentport to complain about Web sites, products, services, or programming that they believe is not appropriate for children.

Concern over what kids are accessing on the Internet is nothing new. For years, groups have said that children should be limited to "clean" sites. But children--both teenagers and even those who have yet to hit that stage in their lives--are trying to access the very content groups like the Mothers' Union are trying to keep them away from.

In 2009, Symantec performed a study on what children were searching for on the Web. And although the first three options--YouTube, Google, and Facebook--were "clean," the fourth-most popular query was "sex." Sixth on the list was "porn."

"I think seeing how dominant the terms 'sex' and 'porn' are, that they come up well within the top 10, doesn't surprise us," Symantec Internet safety advocate Marian Merritt said at the time. "You go down farther in the list, you see words of anatomy like 'boobs,' it almost makes you laugh because we remember what it was like to be a preteen or teen. Parents don't often have context around this sort of thing."

Update at 9:08 a.m. PT to include more details from a subsequent Guardian report.