The government has announced new powers that could force internet service providers to block pornographic websites that do not ask for age verification.
It is part of a wider crackdown on the content that children can view online.
The plan will be included as an amendment to the Digital Economy Bill, the government said.
Critics said it could knock tens of thousands of websites offline despite their content being perfectly legal.
The government has given the British Board of Film Classification (BBFC) new powers to enforce the rules.
Policy shift
Under the proposed legislation, the BBFC would be able to issue a notice to ISPs or mobile operators, asking them to prevent access to websites that contain pornography but have no age-verification system.
In a statement explaining the move, Culture Secretary Karen Bradley said: "The government is committed to keeping children safe from harmful pornographic content online and that is exactly what we are doing.
"Only adults should be allowed to view such content. If sites refuse to comply, they should be blocked."
The rules will apply to all websites in the UK and overseas. Where websites originate in the EU, the process will be compatible with the rules in the country of origin, the government said.