Porn websites must prove users are 18 or risk being banned in UK

The Times is also reporting that "Porn sites will take card details to protect children".

  Pornography websites will be required to install age-verification
  software to help to prevent children from being exposed to hardcore
  images, ministers will announce today.
  
  Users will have to enter their credit card details to prove they are
  over 18, in an attempt to tackle the issue.

https://www.thetimes.co.uk/edition/news/porn-sites-will-take... (paywall)

I've absolutely no idea how this will be enforceable?