Proof of age will be legally required on porn sites under new internet laws.
The shake-up mean that users must be at least 18 years old with their age verification needed before accessing the websites.
The new Online Safety Bill has been set out to ensure all sites publishing pornographic content carry out "robust checks" to guarantee all users are of age.
Verification will be carried out in various ways.
Firms may use age verification technology that asks users to provide details of their credit card to prove their age - credit card holders must be 18 or over.
Third-party services may also be used to confirm someone's age against government data, according to The Mirror.
Campaigners have long demanded the rule to protect children from graphic images and films.
They fear youngsters see a skewed view of sexual norms and practices - warping kids’ minds and harming their relationships in adulthood.
But freedom of speech activists say law-abiding porn enthusiasts could be deterred from browsing pictures and films for fear of their details being discovered and used to blackmail them.
Barnardo’s boss Lynn Perry said: “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.
"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.
“However, this needs to be implemented as soon as possible - the longer we wait, the more children will continue to be harmed.”
The NSPCC’s head of child safety online policy Andy Burrows said: “It’s right the Government has listened to calls to fix one of the gaps in the Online Safety Bill and protect children from pornography wherever it’s hosted.”
Marking Safer Internet Day, Digital Minister Chris Philp said that if sites fail to act, regulator Ofcom will be able to fine them up to 10% of their annual global turnover or block their site in the UK.
Bosses of such sites could be held criminally liable if they fail to cooperate with the watchdog.
Mr Philp said: "It is 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.
"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."
Companies and sites will be able to decide how best to comply with the new rules.
Ofcom may recommend the use of certain age verification technology.
Shadow Digital Minister Alex Davies-Jones said: “ Labour has long called on the Government to offer security for young people online, and we’re glad it has accepted our call for all pornography sites to stop children accessing them by using age-verification technology.
“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 mean that another generation have grown up with access to harmful content online.
“They can and must do better.”
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