The first steps in forcing pornography websites to check that users are over 18 are being announced by the Government this week in an effort to make the internet safer for children.
Websites flouting new rules, which are set to be part of the Digital Economy Act, could find that a regulator has told their internet service providers to block access to them.
Those who provide payment and other services to the site could also be told.
The aim is for all online pornography to have age verification controls by April 2018 along with the appointment of a new regulatory body to oversee and enforce it, the Department for Digital, Culture, Media and Sport said.
It is thought the regulator could be the British Board of Film Classification.
Digital minister Matt Hancock, who is to set to formally kick-start the process with a written statement to the House of Commons on Monday, said: "Now we are taking the next step to put in place the legal requirement for websites with adult content to ensure it is safely behind an age verification control.
"All this means that while we can enjoy the freedom of the web, the UK will have the most robust internet child protection measures of any country in the world."
Online pornography, which can damage a child's development and decision-making, has been seen by 65% of 15-16 year olds and 48% of 11-16 year olds, according to an NSPCC report in 2016.
The study also found that 28% of children may have just stumbled across pornography while 19% had searched for it deliberately.
Will Gardner, chief executive of internet safety charity Childnet, said: "Protecting children from exposure, including accidental exposure, to adult content is incredibly important, given the effect it can have on young people.
"Steps like this to help restrict access, alongside the provision of free parental controls and education, are key.
"It is essential to help parents and carers, as well as young people, be more aware of this risk and what they can do to prevent exposure and also to make sense of exposure if it happens."