Porn websites face being blocked to people in the UK if they fail to check users are over 18, under government plans.
Culture Secretary Karen Bradley said children needed to be safe from "harmful pornographic content online".
The plans would give the British Board of Film Classification (BBFC) powers to block porn sites based in the UK and overseas which did not verify ages.
Critics say the move amounts to censorship of the internet.
Ms Bradley said the step would fulfil a Conservative manifesto pledge to keep children safe.
"Only adults should be allowed to view such content and we have appointed a regulator, BBFC, to make sure the right age checks are in place to make that happen," she said.
"If sites refuse to comply, they should be blocked."
The powers will be set out in amendments to the Digital Economy Bill later this month.
They will allow the BBFC to issue a notice to internet service providers (ISPs) telling them to block access to pornographic websites which have no or inadequate age verification.
Ofcom's guidance on age checks for online video content suggest a range of options - from confirmation of credit card ownership to cross-checking a user's details with information on the electoral register.
The BBFC said any verification mechanism "must provide assurances around data protection" and it would consider those that already and exist and ones currently being developed.
It added: "We will work with all stakeholders to establish the best technological solutions available."
It is understood the government is working with the BBFC to determine the best mechanism that confirms eligibility rather than identifying the user.
The Digital Economy Bill already includes measures to bring in age checks and the power to withdraw payment services from sites which do not implement the controls.
It is thought the measure would also work for free sites which act as teasers for paid-for web pages.
The BBFC said the government's policy would apply to all commercial sites "regardless of where they are based".
A spokeswoman said: "Overseas providers will still be incentivised to comply by the elements of the scheme which will disrupt their income streams and ISP blocking powers greatly increase the chance of effectiveness of the whole regime."
Conservative MP Claire Perry, who has campaigned on internet safety issues, said the move was "absolutely the right thing to do".
She told BBC Radio 4's Today programme: "This was something in the manifesto and it is supported by the overwhelming majority of people.
"Almost 80% of people say of course we want freedom of expression on the internet, of course we want adults to be able to view content whenever they want but just as with the offline world where we don't let children buy porn, we have to have a more measured approach to making sure kids are actually safer online."
'First internet censor'
Research commissioned by the NSPCC and Children's Commissioner for England found more than half of 11 to 16 year olds (53%) have encountered porn online.
The NSPCC said an entire generation of children was at risk of being "stripped of their childhoods" through exposure to pornography at a young age.
Critics of the government plans say the powers will not affect the main commercial porn providers but instead will hit million of other sites which act as their competition.
Jerry Barnett, who used to run a pornography business online and has campaigned against sex censorship, said it could affect sites that simply publish adult material.
He told the Today programme: "This is using pornography as a lever to introduce the first censor of the internet in a democratic country in the world."
"There are already plenty of child protection measures in place. This isn't about protecting children and this doesn't introduce major new protection of children," he added.