A public consultation over plans to implement age checks on pornography websites has been launched by the UK government.
The proposals follow a Conservative Party manifesto commitment that "all sites containing pornographic material" must check that users are over 18.
Internet providers, charities, academics and others will be asked to contribute to the consultation.
A security expert said the plans would struggle to tackle porn on free sites.
In the consultation document, the government proposes that the checks should apply to content that would receive - if formally classified - an 18 or R18 rating from the British Board of Film Classification (BBFC).
"We are keen to hear from parents, schools, child protection experts, the pornography industry, internet service providers and online platforms that provide access to pornographic content," the consultation document explained.
As part of the plans, the government intends to establish a new regulatory framework to enforce compliance with any rules that are made law.
'Matter of urgency'
"Just as we do offline, we want to make sure children are prevented from accessing pornographic content online which should only be viewed by adults," said Baroness Shields, the Internet Safety and Security Minister.
Peter Wanless, chief executive of the NSPCC said the issue was "a matter of urgency" and that children who had ready access to content rated over 18 could develop a "warped" view of sexual relationships.
Prior to the general election, then Culture Secretary Sajid Javid said that the appointed regulator would have the power to force internet service providers to block sites that did not perform effective age checks.
He also said providers who did not cooperate could be fined.
The consultation document explains that the regulator of age checks on porn sites could be given powers to impose "sanctions" - including fines.
Companies which support or provide services to the producers of pornographic content online could be "directed" to withdraw those services if sites were found to be persistently non-compliant.
This could involve directing payment firms to cease processing transactions for porn producers.
How would age checks work?
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.
"Age checking is nothing new," said Ben Jordan from Aristotle, which provides such services.
"The online gambling industry has been doing this for years and it's been very effective."
Dr Victoria Nash, at the Oxford Internet Institute, was the lead author of a report for the government on child use of online pornography.
She said that gambling sites may well also check registered users' credentials against data in the electoral roll or driving licence databases.
Dr Nash added that it was likely porn site age checks would use similar methods to ensure users are 18 or over.
Free porn concerns
However, a leading security researcher has said this would not stamp out much of the pornography that is freely available online.
"My initial response to this release is that while the UK government may get some traction with payment processors, and that that could have some effect, it won't affect the free pornography," said Dr Gilad Rosner, who is a member of the Cabinet Office Privacy and Consumer Advocacy Group.
Dr Rosner added that much pornographic content was available on free sites and blogs which do not take any payments from users.
"I would wager that we won't see much in the way of tangible results for the next five years," he said, "This is a particularly challenging regulatory goal."
Even so, one of the most visited free sites for pornography has already signalled its intention to co-operate.
"Pornhub will comply fully with any robust legislative and relevant local measures that emerge from this consultation," said a spokeswoman for the site.
"As members of the Digital Policy Alliance, we have already been working to carve out a technical solution which works for all.
"It's essential that all explicit adult content is covered, and non-compliant sites are quickly taken down."