Ofcom told the BBC it had never heard from a porn company fined £1 million for failing to comply with the UK's Online Safety Act.
The company said it had emailed AVS Group Ltd since the investigation began in July but had not received a response, so the firm was fined an additional £50,000.
The law creates a legal requirement for websites hosting pornographic material to implement what the regulator defines as “highly effective age guarantees” to prevent children from easily accessing explicit content.
Ofcom said AVS must now implement high-impact age enforcement within 72 hours or face an additional fine of £1,000 a day.
In addition to the AVS fine, Ofcom also announced that one “major social media company” is undergoing a compliance process with its enforcement team.
The regulator has not named the platform but says formal action could be taken if sufficient improvement is not achieved soon.
Ofcom said the fine showed the “tide in online safety” was beginning to turn.
“There have been important changes for people this year, with new measures on many sites and apps now better protecting children from harmful content,” said Oliver Griffiths, director of Ofcom's online safety group.
“But we need to see a lot more from tech companies next year and we will pull out all the stops if they don't live up to expectations,” he added.
Ofcom has already started fining some companies for failing to provide adequate age verification. including deepfake nudity apps.
However, on the online message board 4Chan still refused to obey Ofcom imposed a £20,000 fine in the summer.
The Online Safety Act is being implemented in stages and aims to prevent past practices that Ofcom described as online platforms being “unregulated, unaccountable and often unwilling to put people's safety before profit”.
Stricter age checks for porn sites were introduced in July, although some say they can be easily avoided by using a virtual private network (VPN) that redirects Internet traffic.
Pornhub's parent company told BBC News about the matter in October. saw a 77% drop among UK visitors since age verification was introduced.
Baroness Beeban Kidron, founder of the 5Rights Foundation, told the Today program that fines are “nothing” for tech companies.
“Business disruption is everything,” she said.
“If we are not prepared to use the law, they are not actually doing what parliament is asking them to do.
“We need a completely different attitude from the regulator in terms of intensity and reliability to say: we have the law and we are using it.”
The BBC contacted TubeCorporate, the adult content publishing platform behind the AVS group Ltd websites, for a response.
The address the firm uses is in the Central American country of Belize and appears to be the registered address of a large number of companies, although they do not have physical offices there.
Tighter guidelines were also introduced this year keeping the internet safe for women and girlswith Ofcom vowing to name and shame platforms that fail to comply.
Critics say the law needs to be strengthened to make the internet safer, especially for women and girls.
