An age-old screening scheme designed to prevent under-18s from viewing pornographic sites will take effect on July 15.
From that date, affected sites will have to check the age of UK visitors.
If they do not comply, they will be blocked by internet service providers.
But critics say teenagers may find it relatively easy to circumvent the restriction or simply use pornography shelters not covered by the law.
Twitter, Imgur and Reddit, for example, will not be required to administer the scheme because they fall under one exception where more than one-third of the content of a site or application must be pornographic to qualify.
Likewise, any platform that hosts pornography, but does not do so on a commercial basis – meaning it does not charge a fee or make money from ads or other activities – will not be affected.
In addition, it will still be legal to use virtual private networks (VPNs), which may make it appear that a UK based computer is located elsewhere to escape age checks.
"The introduction of mandatory age verification is a global innovation and we take the opportunity to balance privacy concerns with the need to protect children from inappropriate content," said Digital Minister Margot James.
"We want the UK to be the safest place in the world to be online, and these new laws will help us achieve that."
Call to action
Authorities acknowledged, however, that age verification is not a "silver bullet" solution, but a means of making it less likely that children stumble into inappropriate materials online.
However, the government said other measures could follow.
"We know that pornography is available on some social media platforms and we expect these platforms to do much more to create a safer environment for children," a spokesman for the Department of Digital Media, Media and Sport ).
"If we do not see the action, we do not rule out in the future legislation to force companies to take responsibility for protecting vulnerable users from the potentially harmful content they host."
The age checks were originally proposed by the now defunct regulator Atvod in 2014 and were enacted as a law in the Digital Economy Act of 2017. But the implementation of them was repeatedly postponed.
Pornographic video services hosted in the UK already need to check the age of the visitors, as well as the online gambling platforms.
Porn passes & # 39;
The British Board of Film Classification (BBFC) – which gives the films their old certificates in the UK – will be responsible for regulating the effort. It will instruct internet providers about which sites and applications block non-compliance. In addition, you can ask payment service providers to get support and request that search engines and advertisers avoid an offending company.
Pornographic platforms themselves will be free to choose how to check the ages of UK visitors.
But the BBFC said it will award solutions that adopt "robust" data protection standards with a certificate, allowing them to display a green AV (age verification) symbol in their marketing materials to help consumers make an informed choice.
Mindgeek, one of the biggest players in the adult industry, has developed an online system called AgeID, which it hopes will be widely adopted. It involves adults who need to upload scans of their passports or driver's licenses, which are then verified by a third party.
Digital rights advocates at the Open Right group have raised concerns over the idea of entrusting these companies with personal data from users. But Mindgeek said that all information will be encrypted and that the AgeID system will not track how each user's accounts are used.
Street stores and newsstands also sell separate age verification cards to adults after conducting face-to-face checks, according to the government.
Nicknamed "porn passes" by the media, the idea is for users to enter a code printed on the cards on porn sites to gain access to their content.
The BBFC said it will also create an online form for members of the public to mark sites that do not comply when the new regulations take effect.
Age checks are part of a broader effort by the UK authorities to make the internet safer for young people.
More recently, DCMS has proposed the creation of a new regulator to handle applications that contain content that promotes self-harm and suicide, among other problems.
In addition, the Information Commissioner's Office has proposed that services cease using tools that encourage minors under 18 to share more personal information about themselves than they would otherwise.