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.
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.
The Internet Service Providers` Association (Ispa) said in a statement: ""Moving to blocking legal adult content represents a significant policy shift that could have far-reaching implications for the UK Internet.
"Government must clearly assess and quantify the impact of industry in terms of competition, innovation and investment, put in place a robust regulatory system and address the potential for unintended consequences, including existing industry self-regulatory efforts to tackle child sexual abuse material, scope creep and over-blocking."
Jim Killock, executive director of the Open Rights Group added: "In no way should this proposal be legislated for in this Bill. There has been no thought or consultation, and the government has not even begun to define how blocking might be attempted.
"They have no idea if it would work well or badly, or whether there is serious enough harm to justify such a massive restriction on UK adults` access to legal material."
The Digital Economy Bill already contains measures to bring in age verification for pornographic sites and the ability to withdraw payment services from sites that do not comply.
The government said its new rules were based on studies that suggest viewing pornography at a young age can have detrimental effects on future adult relationships.
A survey from the NSPCC found that nearly half of 11-16-year-olds had accessed an adult site and one in five 11-17-year-olds said that they had seen images that had shocked or upset them.
At present, all of the UK`s biggest ISPs and mobile operators have a voluntary approach to blocking adult sites. Parental controls give subscribers a choice about whether to limit access to such sites.