It’s often said pornography is a hidden driver of technological change, thanks to the industry’s readiness to adapt to market trends. As one story goes, the wider selection of porn on VHS was the decisive factor in its dominance over Betamax in the 1980s.

Soon, the porn industry will be forced to adapt again – this time to a new regulatory regime in the United Kingdom. By the end of 2018, all commercial porn sites will have to find a way to “effectively verify” that their users in the U.K. are over 18 years old, and block access to everyone else. That doesn’t mean just ticking a box – it means advanced verification systems where online porn users must prove their age.

The U.K. government says the Digital Economy Act, passed last year, will make the country the safest place in the world for children to be online. But some free speech advocates and pornographers have voiced fears that the measures are the first step towards censorship of the Internet, while privacy experts say the move could put too much personal data into the hands of large adult film companies, a tempting target for hackers. Once again, pornographers will be at the vanguard — this time, in a war about free speech and access to information that could have global repercussions.

If the new technologies being developed to implement the block work well, there’s a chance they could spread rapidly around the globe as governments seek to put controls on otherwise unregulated online space. When that happens, porn might not be the only thing getting blocked. “This is the first example in a western country of an official state Internet censor being introduced,” Jerry Barnett, a campaigner for free speech and sexual freedom, tells TIME. “The fact that their first power relates to porn sites is less relevant.”

Barnett worked in the adult film industry until 2012, and is the author of Porn Panic! Sex and Censorship in the U.K. “From the very start,” he says, “I didn’t see this as about porn. I saw this as a strategy that had been formulated to censor the Internet, and porn had been picked as the excuse to do it.”

Once the U.K.’s block is in place, other types of legal content could be put behind barriers or even removed from the Internet, according to the non-profit Open Rights Group, which campaigns for digital civil liberties. It warns that websites containing content on suicide, anorexia or unpalatable political views could be next. That, it says, “poses a major threat to the free speech of U.K. citizens.”

But one in five British children aged 11 to 17 said they had seen pornographic images that had shocked or upset them, according to a study cited by government. Meanwhile, the argument goes, access to sexually explicit content is regulated in all other areas of British life, from films to video games to top-shelf magazines. Why should online life be any different?

“At the moment, pornography is only one click away for any child in the U.K.,” says David Austin on a recent morning in London. He’s the chief executive of the British Board of Film Classification (BBFC), the government-appointed body that, starting later this year, will decide which sites must verify the ages of their visitors. He’s adamant that the block isn’t the start of a slippery slope to a censored Internet.

“It’s a child protection measure,” says Austin. “What it isn’t is censorship.” Adults will be able to access the same legal material they’re entitled to already, he says, but will just have to prove their age. His position is clear: no rights are being infringed.

His organization is an old gatekeeper of public morality. Since 1912, it has been deeming which content is appropriate for consumption by Brits. From its inception a private body, responsible for its own funding, it currently holds government authorization to decide movie certificates, and already determines which websites can (and can’t) be accessed over mobile connections by many of Britain’s 48 million smartphone users. Now, in an unprecedented expansion of its official remit, the BBFC is in the process of deciding which of the 5 million commercial porn sites it estimates exist on the Internet must verify the ages of their users.

But unanswered questions remain about how effective the law will be at tackling smaller sites, and others where the line is blurred — for example, sites that host erotic art, sex education resources, or non-commercial porn. “It’s not a silver bullet,” Austin admits.

That means somewhere, somehow, a line will have to be drawn through the gray area. Austin says it won’t be difficult. The BBFC has been applying a “tried-and-tested” definition of pornography since the 1980s — “content whose primary purpose is sexual arousal,” he says. “We know what we’re doing. We know what’s art and what’s porn.”

The U.K.’s first female porn director is unconvinced. Now a lecturer in gender studies, in the late 1990s and early 2000s Dr. Anna Arrowsmith found herself at frequent loggerheads with the BBFC over what she saw as its censorship of her late-night TV porn films – the first in the U.K. to be made for a female audience. “The porn block is a massive piece of censorship,” she says over the telephone from her home in California. “Because it says that porn isn’t a real piece of culture. Actually, it’s a very popular piece of culture.”

Arrowsmith’s concerns about age verification come from a deeply-held conviction. “The porn industry is sexist,” she says. “But the main way to deal with that is to get more women and different types of voices in there. As soon as any legal or financial block comes in your way, you’re going to be the one that goes first.”

But doesn’t she think stopping children from accessing adult material is a good idea? “I don’t think that we should be doing stuff that affects the whole of the country and damages women and minorities’ explorations of their own sexuality,” she says. “I don’t think we should be getting rid of all that just for kids.”

The nakedness of that argument shows the trouble activists are having. Protecting children from stumbling across depraved material has greater appeal than the often hazy counterarguments of slippery slopes and safeguarding civil liberties. “We protect [children from] television … and we protect them in the cinema,” said Maria Miller, a lawmaker in the ruling Conservative Party, debating the law in Britain’s parliament. “In one of the most uncontrolled environments —online—we allow them freely to view things that are far more difficult for us as parents to control.”

Indeed, some of the most ardent critics of the block who TIME spoke to still agreed that young children should, somehow, be protected from accessing pornography. The core of the dispute is over how to do it, and what the side-effects will be.

Under the 2017 Digital Economy Act, the U.K. government left the design of age verification technologies to the market. MindGeek, a market leader and the owner of many of the Internet’s biggest porn websites, has developed its own tool called AgeID, which it says it will make available to independent porn producers in the U.K. for free (though its competitors will have to pay).

Suggestions of how other systems might work include credit-card verification, or buying an age-restricted access code in a store in the same way one might buy cigarettes or alcoholic drinks. The point, Austin says, is that viewers’ identities don’t have to be directly associated with a token proving they’re over 18. In theory, the free market should allow systems that protect users’ privacy most effectively to thrive.

But critics are still concerned about the privacy implications of associating one’s identity with his or her private sexual fantasies, even indirectly. Age verification will enable “the detailed tracking of all habits — what people watch, when, where and for how long,” says the Open Rights Group in a statement. Those habits, it says, will be linked to a unique identifier, “creating a new and unique privacy risk.”

Such technologies, the Group says, are at risk of a large sensitive data breach; unprecedented data collection across multiple sites on the sexual preferences of users; the sharing of login information among teenagers (potentially leading to bullying or the outing of LGBTQ teenagers); and even the increased risk of grooming if teenagers develop a relationship with an adult in return for a login. And in an age of sophisticated personal profiling, there are fears that allowing shady adult websites to control such tools leads to a conflict of interest between the user (who seeks privacy,) and the company (which has the incentive to collect data).

Nevertheless, a leading British children’s charity says the Digital Economy Act is not going far enough. According to the text of the law, only pornographic websites operating “on a commercial basis” will require age verification measures, leaving swathes of adult content unregulated. “We were disappointed that social media sites were excluded from the legislation,” says Abbie Gillgan of the National Society for the Prevention of Cruelty to Children. “We know that children and young people are often exposed to pornography and other inappropriate content on social media platforms, and we’ve seen time and again how these sites are failing to do enough to protect children.”

The U.K.’s age verification measures were scheduled to come into force in April of this year. But in March, buried deep within a government document about the rollout of 5G Internet, it was announced that the deadline will be pushed to the end of the year. Lawmakers still have to ratify the proposals, and with Brexit eating up parliamentary time, that seems unlikely to happen soon. There is also a legal provision for a three-month period between parliament approving the bill and it coming into force, leaving time for a legal challenge.

One thing that seems unlikely in the U.K. is much public discussion over the law, even if it does come into force by the end of 2018. That’s because the risk of being characterized as a pervert pervades discussions of the topic. “People like Jerry and any other adults who want to watch pornography will continue to be able to watch pornography just as they do now,” says David Austin. “Provided it’s legal.”

Correction: August 22

The original version of this story misstated whether online porn users must prove their identity. They must prove their age, not their identity. The original version of this story also misstated whether the BBFC determines which websites can be accessed over mobile data in Britain. It does, but users can opt out of the regulation.

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Write to Billy Perrigo at billy.perrigo@time.com.