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.