Ofcom has published what it calls a "non-exhaustive" list of technologies that may be used to verify ages, which includes:
The rules specifically state that "self-declaration of age" is no longer considered a "highly effective" method of checking ages - and therefore is not acceptable.
It also states that pornographic content should not be accessible to users before they have completed an age check.
Other age verification firms have responded positively to the news.
"The regulator's long-awaited guidance on age assurance means adult content providers now have the clarity they need to get their houses in order and put in place robust and reliable methods to keep explicit material well away from underage users," said Lina Ghazal, head of regulatory and public affairs at Verifymy.
But privacy campaign group Big Brother Watch warned that many age-checking methods could be circumvented, and should not be seen as a panacea.
"Children must be protected online, but many technological age checking methods are ineffective and introduce additional risks to children and adults alike including security breaches, privacy intrusion, errors, digital exclusion and censorship," said boss Silkie Carlo.
"We must avoid anything like a digital ID system for the internet that would both eradicate privacy online and fail to keep children safe," she added.