Hardcore pornography websites will be forced to put in place age-restriction controls or be shut down if the Tories win the election.
Culture Secretary Sajid Javid said the party would act to ensure under-18s were locked out of adult content.
His promise comes after a Childline poll found nearly one in ten 12 to 13 year olds are worried they are addicted and 18% have seen shocking or upsetting images.
Experts welcomed the move but warned it would take hard work to implement.
Under the proposals the system would be overseen by an independent regulator with the power to compel Internet Service Providers (ISPs) to block sites which failed to include effective age verification.
ISPs could be fined if they did not co-operate.
Mr Javid said: “If you want to buy a hardcore pornography DVD in a store you need to prove your age to the retailers.
“With the shift to online, children can access adult content on websites without restriction, intentionally or otherwise.
“As a father to four young children, I worry, like every other parent, how easy it is for them to view explicit material.
“That is why we need effective controls online that apply to UK and overseas.”
Mother’s Union chief executive Reg Bailey, who was commissioned by David Cameron to investigate the sexualisation of childhood, said it was “a really welcome development”.
He added: “A key recommendation of the Letting Children Be Children review was to help parents protect children from inappropriate content on the internet especially pornography.
“Although huge progress has been made, this is a real step forward.”
Sarah Green, of the End Violence Against Women Coalition, said: “We welcome proposals to better regulate access to online pornography.
“Surveys have shown more than half of young people have seen online pornography by the age of 14, and that many see it without even seeking it out as links are shared on social networks.”
Mr Javid said an effective age control mechanism might be the use of credit cards.
He added that in some countries, such as Finland, there are electronic IDs in place.
Labour culture spokesman Chris Bryant said: “After five years of inaction by the Tories, this proposal is too little too late. Protecting children from inappropriate material both on and offline should be a priority but the Tories have failed to act quickly enough.”
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