A senior US investigator says it is still easy to find child abuse images that have originated on OnlyFans.
The online platform told the BBC it had acted to tackle illegal content and it was impossible for them to verify claims without evidence.
But an investigator said images of young children apparently being exploited could be traced to the site.
A BBC News investigation last year revealed children had sold and appeared in videos on the British website.
Leaked documents also showed moderators were allowed to give multiple warnings to accounts posting illegal content before deciding to close them.
Now, the company's bosses told BBC Newsnight they had taken firm action following the investigation.
Amrapali Gan was made chief executive of OnlyFans last year. She says the company is the safest social media site in the world.
"We actively work with law enforcement. If anyone makes the mistake thinking they can upload illegal content, we will report them," she says. "We're truly the safest and most inclusive social media platform."
However, Newsnight has found further evidence that the site is failing to prevent illegal content from appearing on its site.
A US investigator says that in just an hour they were able to find ten child abuse images on other platforms that had originated on OnlyFans. They believe the photos have been created within the last six months and include watermarks from the website.
Some images show pre-pubescent children being directed to produce abusive images, according to the agent, who specialises in investigating paedophile rings online.
"The youngest was around five years old," the investigator told the BBC, adding that others were about 12 years of age. "Whatever their [OnlyFans'] current methodology, there's still cracks that it's still slipping through", they added.
In a statement, OnlyFans said: "When the BBC raised this anonymous claim, we asked them for evidence to enable us to investigate."
It added: "The BBC refused to provide any details or evidence preventing OnlyFans from investigating this claim."
The BBC has agreed not to identify the agent who redacted account usernames to protect their investigation.
The NSPCC said a small number of reports made within the last year to its counselling service, Childline, also suggested OnlyFans continued to host images of child abuse. It said one mother claimed that images of her 10-year-old daughter had been uploaded to the site.
"Children are not only consuming content on OnlyFans, but also are able to produce content", says Hannah Ruschen from the NSPCC.
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A US charity also recently discovered that language banned by the site was still widely in use, including a term referencing young girls.
OnlyFans said a "glitch" enabled some text to be ported from other websites and it had now removed the term highlighted by the Anti-Human Trafficking Intelligence Initiative.
The company does not believe there is a significant amount of illegal content on its site and said it reviews every piece of content posted on its platform, including video, photos and text.
"We've invested significantly in improving our age and identity verification tools", said Keily Blair, chief strategy officer for OnlyFans.
"One of the issues that was raised was being able to pass off the account that you've opened to somebody else, that's not now possible."
Ms Blair said OnlyFans does more to address underage use than anyone other social media site. "The [rest of] the industry needs to do better around online safety", she added.
Founded by Essex businessman and former chief executive, Tim Stokely, in 2016, OnlyFans has since grown significantly. Its users spent more than £4bn on purchasing photos and videos shared on the site, last year.
Ms Gan said 80% of the company's staff work on content moderation and it would continue to invest in this area. She also said the website's decision, last year, to ban pornography – which was quickly reversed – was a "learning experience".
OnlyFans is not alone in taking steps to address online safety. The government's long-delayed online safety bill proposes heavy fines for websites failing to protect children.
Last year, it also provided Ofcom with limited powers to penalise companies which are allowing harmful content on video-sharing platforms. But Ofcom's role does not involve assessing individual videos and no companies have been sanctioned to date.
The NSPCC said neither the current regulation nor the planned legislation go far enough. "We want to see senior manager liability when Ofcom identifies failure to comply with the children's safety duties, rather than just failure to comply with information requests", added Ms Ruschen.
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