Online pornography sites could face fines of up to £250,000 unless they take action to prevent under-18s viewing explicit material, under Government proposals.
New laws will require adult websites to have some form of age verification technology to prevent children accessing them.
Ministers highlighted research showing 1.4 million under-18s visited adult sites from their desktop, roughly a fifth of children accessing the internet in the UK.
Internet Safety and Security Minister Baroness Shields said: "The internet is a tremendous resource for learning and creativity but it is important to make sure that children are able to make the most of all it has to offer in a safe way. Keeping children safe online is one of government's greatest priorities.
"Just as we do offline, we want to make sure children are prevented from accessing pornographic content online which should only be viewed by adults."
The consultation follows a promise to act in the Tory manifesto and Lady Shields said the proposals would put the responsibility on pornographers to ensure they have safeguards in place to block under-18s.
Alongside the legal requirement for companies to put in place age verification technology, the Government will also establish a new regulatory framework to monitor and enforce compliance, with the ability to impose fines where breaches are identified.
In a signal of the level of fines that could be imposed, the consultation points out that existing regulations governing video-on-demand services can result in a maximum fine of £250,000.
NSPCC chief executive Peter Wanless said: "This consultation is an important and welcome step forward in keeping children safe from online pornography. Companies that produce and profit from this material have a responsibility to give children the same protection they would get in the offline world.
"Every day ChildLine receives calls and messages from young people who feel they are being badly affected by the way they and their friends can view unlimited online pornography.
"As a matter of urgency we must prevent children having ready access to content rated over 18 as it can give them a warped view of sexual relationships."
The consultation, which runs until April 12, highlights figures from the May 2015 comScore study which showed 1.4 million unique visitors aged under-18 accessed adult sites featuring violence or pornography, roughly 20% of under-18s with internet access.
Some 13% of of children aged six-14 visited an adult site in May 2015, the study showed.
Under the new proposals the new regulatory regime will also allow credit card firms and advertises to withdraw services from porn sites found to be in breach of the law.