Porn sites will have to ensure users users are 18 years old or over, under plans announced by the government on Tuesday.
The online safety bill will be expanded to legally require “robust checks” are made by websites.
Chris Philp, the digital minister, said it was “too easy for children to access pornography online”.
“Parents deserve peace of mind that their children are protected online from seeing things no child should see,” he said.
“We are now strengthening the online safety bill so it applies to all porn sites to ensure we achieve our aim of making the internet a safer place for children.”
Under the plans, Ofcom will be able fine sites up to 10 per cent of their annual worldwide turnover or can block them from being accessible in the UK if they fail to act.
The legislation had initially only targeted commercial porn sites that allow user-generated content.
Now all commercial porn sites are now within the scope of the proposed new rules.
Age verification could take the form of a credit card check or details that the user’s mobile provider holds.
But the government said sites should not process or store data that is irrelevant to the purpose of checking age.
Alex Davies-Jones, shadow minister for tech, gambling and the digital economy, said Labour had “long called” for security for young people online.
“We’re glad it has accepted our call for all pornography sites to stop children accessing them by using age-verification technology,” she said.
“We need strict age protection rules, while tightly regulating age-verification tech to ensure they are not being used to collect unnecessary personal data and to protect people’s privacy online.
“Ultimately the Tories’ persistent delays on online safety legislation means that another generation have grown up with access to harmful content online – they can and must do better.”
Lynn Perry, the interim CEO of Barnardo’s, said the charity was “delighted” at the move.
“We know from our direct work across the UK that exposure to pornography has a hugely harmful impact on children’s mental health, body image and their understanding of healthy relationships,” she said.
“As we have explained to ministers, children are stumbling upon violent and extreme pornography as young as six years old and these measures will finally help to stop that happening.”