Online marketplace Vinted says it has removed sexually explicit adverts, after a mum reported seeing a video depicting what she believed to be a pornographic scene while browsing for clothes.
Kirsty Hopley, 44, from Carlisle, said she was searching the app for a dressing gown when the ad popped up.
She was sitting next to her teenage daughter at the time.
Ms Hopley reported the content to Vinted and later contacted Ofcom.
She told BBC News the video, which started playing automatically, showed a “sickening” graphic and violent sexual encounter.
The law and criminology teacher said she had installed content filters on her home internet and was shocked to see such material on the e-commerce platform.
“I probably won’t buy anything from there again, which is disappointing as I love Vinted,” she said. “But I don’t want to see content like that.”
The platform, which has no age restrictions, has recently faced scrutiny in France after reports that some sellers were using the site to direct users to adult content.
The advert Ms Hopley saw was promoting DramaWave, a mobile app that produces short-form scripted stories for social platforms.
Many of their series appear to follow romance storylines over multiple episodes of just a few minutes each.
Vinted said the adverts have now been blocked.
A spokesperson said the platform has a “zero-tolerance policy for unsolicited sexual communications and the promotion of sexual content”.
“This includes prohibiting sexually explicit advertisements on our platform,” they said.
“Where listings or ads are found to violate these rules, we will take action, including blocking or removing them.”
The BBC has approached DramaWave for comment.
The Advertising Standards Authority (ASA) told BBC News their rules were clear “that ads must not cause harm or offence”.
“Harmful or degrading portrayals of women in ads are completely unacceptable, and we take a zero-tolerance approach to this kind of content,” they said in a statement.
They added: “We encourage anyone with concerns about an ad they’ve seen to get in touch.”
Ms Hopley told BBC News she expected the UK’s Online Safety Act (OSA), which includes laws to protect children from explicit content online, to have prevented such material appearing on her phone.
However the only paid-for advertising that is in scope of the OSA is fraudulent content.
