Websites without age-verification software may have to pay the eye-watering fine or even be banned altogether.
And those with the software will have to check visitors' credit card details so as to confirm their ages.
The Government will also announce plans to appoint a regulator that will police the adult websites.
It's believed that British Board of Film Classification – which sets age limits on films – will get the job.
The new government powers will apply to both free and paid-for adult websites, as well as websites that are hosted abroad.
Digital Minister Matt Hancock said: "We are taking the next step to put in place the legal requirement for websites with adult content.
"All this means that while we can enjoy the freedom of the web, the UK will have the most robust internet child protection measures of any country in the world."
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Will Gardner, chief executive of internet safety charity Childnet International, said it was an "essential" measure to protect youngsters on the Internet.
"Protecting children from exposure, including accidental exposure, to adult content is incredibly important, given the effect it can have," he said.
"Steps like this to help restrict access, alongside the provision of free parental controls and education, are key. It is essential to help."
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