Media regulator Ofcom said it fined AVS Group Ltd, an operator of 18 adult websites, for lapses relating to age checks plus an extra Stg50,000 for failing to respond to information requests.
AVS did not immediately respond to a Reuters request for comment.
The Online Safety Act is designed to protect children and vulnerable users from illegal content online and came into force earlier in 2025, but has caused tensions between US tech companies and Britain.
Critics of the law have said it threatens free speech and targets US companies.
This is the third company Ofcom has fined under the new regime.
The first was US internet forum site 4chan in October for Stg20,000.
Ofcom said it started looking at AVS in July as part of an investigation into dozens of adult sites which have millions of monthly visitors in Britain.
It said it had issued the fine because it did not consider AVS's age verification process to be "highly effective".
The penalty AVS faces could increase by Stg1000 a day if effective age checks are not brought in within 72 hours.
"We continue to investigate other services' compliance with age check requirements and will take action where necessary," Ofcom said in its statement.
