Playboy has been fined £100,000 by Ofcom for its lack of safeguards on Playboy TV and Demand Adult, two of the company's pornographic websites.
The two sites were judged to have failed to protect children from "potentially harmful" pornographic material.
They allow users to access hard-core pornographic images and videos without having enough controls in place to ensure users were aged 18 or over.
Ofcom, the independent communications regulator, does not normally regulate porn websites.
However, Playboy TV and Demand Adult both provide access to videos in a way similar to that in which adult services are broadcast on television - therefore they fall within Ofcom's jurisdiction.
It said: "Ofcom concluded that Playboy's failure to protect children from potentially accessing these sites was serious, repeated and reckless.
"The homepage of Demand Adult displayed hard-core pornographic material that could be viewed by clicking on a button labelled 'Enter I am over 18'. In order to access additional paid for content, users could pay using a debit card. Neither of these controls constitutes an effective age verification system.
Playboy adverts banned
"Playboy TV also required users to self-certify their age before accessing the main site; however unlike Demand Adult, the material on the homepage showed sexual activity without explicit detail.
"In order to access the website's hard-core pornographic material users could register using a debit card, which is not an effective age verification system."
Ofcom said there are a number of ways websites can verify the age of its users, including asking for credit card details before adult content is made available - credit cards, unlike debit cards, are only available to adults.
As a result, Ofcom fined Demand Adult £65,000 and Playboy TV £35,000.
In December, Ofcom fined Playboy's movie website Strictly Broadband £60,000 for failing to implement effective controls to stop children accessing its content.
The month before, the Advertising Standards Agency had banned one of Playboy TV's adverts that appeared on a taxi and a lorry for being overtly sexual.
The ASA judged: "The ads were in an untargeted, mobile medium and therefore had the potential to be seen by a large number of people who were likely to find the images of scantily clad women in overtly sexual and provocative poses, offensive.
"We considered that, because the pictures were overtly sexual and could be seen by anyone including children, they were likely to cause serious and widespread offence and were irresponsible."