Four years after the country began crusading against online porn sites, the United Kingdom says it will require mandatory age verification for websites that host adult content. The rules are expected to go into effect by April 2018.

Digital Minister Matt Hancock called the signing of the Digital Economy Act 2017 “a milestone in the Government’s work to make the U.K. the safest place in the world for children to be online.” The act addresses policy issues related to electronic communications. Beyond its efforts to stem underage viewing of online porn, it also will require TV and video on demand services to provide subtitles and audio descriptions and make it a criminal offense to use bots to buy up online tickets and sell them at inflated prices.

The move, while drastic, is a notable step back from a pledge by then-prime minister David Cameron’s attempt in 2013 to block online porn, unless people chose to opt in to receive it. That proposal was quietly abandoned amid concerns regarding the possible invasions of user privacy.

While no one would argue that children should be exposed to online porn, there are still a number of questions about how effective these age gates will be. The details of the plan haven’t been announced and even Hancock admitted in a written statement that “the new scheme is complex.”

And that doesn’t begin to address the volume and persistence of sites that host the material.

Globally, porn is a $97 billion industry, according to Kassia Wosick, assistant professor of sociology at New Mexico State University. At present, between $10 and $12 billion of that comes from the United States, but it’s harder to determine how big a percentage of the overall market is represented by the U.K.

A 2006 study by Nielsen NetRatings, however, found Britain to be the fastest growing pornography market in the world—with almost 40%of the country’s male population visiting porn sites throughout the year. That’s due in part to the U.K.’s already stringent policies on adult content. (Television is restrictive on what can be shown and the DVDs sold are often the censored version.) For people wanting to see hardcore porn, the web is often the best option.

The effectiveness of the age gates will depend on a few factors. Simply asking the user to vouch they’re over 21 won’t stop many kids. But requiring some form of identification, like a credit card or government ID, will likely be met with legal challenges.

And that’s assuming the government and ISPs in the U.K. can effectively filter adult sites. Several years ago, Ogi Ogas and fellow neuroscientist Sai Gaddam, co-authors of A Billion Wicked Thoughts (a book that studied the sexual behavior of more than a hundred million men and women around the world by observing what they do when taking advantage of the Internet’s anonymity), tried to identify every piece of porn online. They gave up, said Ogas, after finding it was simply too widespread to track.