Mininister have announced it'll bring about the change from next Spring.
Under new powers contained in the Digital Economy Act, which passed through Parliament earlier this year, a regulator will be able to block porn websites that fail to show that they are denying access to under-18s, for instance by demanding credit card details.
Digital economy minister Matt Hancock has now signed an order allowing the designation of the regulator, expected to be the British Board of Film Classification, to put the new protections into effect from April next year.
The announcement has been welcomed by the Christian charity CARE.
Chief Executive Nola Leach said: "Children deserve the strongest possible protections online and the robust age-verification checks on pornographic websites, brought in through The Digital Economy Act, are a vital milestone in the fight to stop children being exposed to harmful content online.
"The distinction between the offline and online world becomes less apparent almost daily in our fast-paced digital age. In the offline world children are shielded from harm through age rating mechanisms for films and magazines and a host of other tools - age-verification for access to pornographic content is a very welcome step towards bringing protection of children online into step with the offline world.
"The internet is a valuable resource, but it can also be dangerous if not used safely or in an age-appropriate way. These new measures provide an added layer of protection for children and welcome peace of mind for parents concerned about their children accessing explicit material online."
Leach said there were some concerns that remained.
"There is still more work to do," she said. "For example, not all explicit and graphic material will be placed behind age-verification checks. A new Bill introduced in the House of Lords by Baroness Howe of Idlicote aims to tie up some of the loose ends in the Digital Economy Act by addressing some of the definitions of the type of content which should only be accessible behind age-verification.
"This Bill strengthens the Digital Economy Act and should be welcomed by the Government and anyone who wishes to protect children from accessing harmful inappropriate content online."