One night in 2023, Eric was scrolling on a social media channel he regularly browsed for porn. Seconds into a video, he froze.

He realised the couple he was watching - entering the room, setting down their bags, and later, having sex - was himself and his girlfriend. Three weeks earlier, they had spent the night in a hotel in Shenzhen, southern China, unaware that they were not alone.

Their most intimate moments had been captured by a camera hidden in their hotel room, and the footage made available to thousands of strangers who had logged in to the channel Eric himself used to access pornography.

Eric (not his real name) was no longer just a consumer of China's spy-cam porn industry, but a victim.

Warning: This story contains some offensive language

So-called spy-cam porn has existed in China for at least a decade, despite the fact that producing and distributing porn is illegal in the country.

But in the past couple of years the issue has become a regular talking point on social media, with people - particularly women - swapping tips on how to spot cameras as small as a pencil eraser. Some have even resorted to pitching tents inside hotel rooms to avoid being filmed.

Last April, new government regulations attempted to stem this epidemic - requiring hotel owners to check regularly for hidden cameras.

But the threat of being secretly filmed in the privacy of a hotel room has not gone away. The BBC World Service has found thousands of recent spy-cam videos filmed in hotel rooms and sold as porn, on multiple sites.

Much of the material is advertised on the messaging and social media app Telegram. Over 18 months, I discovered six different websites and apps promoted on Telegram. Between them these claimed to operate more than 180 hotel-room spy-cams which were not just capturing, but livestreaming, hotel guests' activities.

I monitored one of these websites regularly for seven months and found content captured by 54 different cameras, with about half operational at any one time.

That means thousands of guests could have been filmed over that period, the BBC estimates, based on typical occupancy rates. Most are unlikely to know they have been captured on camera.

Eric, from Hong Kong, began watching secretly filmed videos as a teenager, attracted by how "raw" the footage was.

"What drew me in is the fact that the people don't know they're being filmed," says Eric, now in his 30s. "I think traditional porn feels very staged, very fake."

But he experienced what it feels like to be at the opposite end of the supply chain when he found the video of himself and his girlfriend "Emily" - and he no longer finds gratification in this content.

When he broke the news to Emily that their hotel stay had been filmed, edited into an hour-long clip, and uploaded to Telegram, she thought he was joking. But then she saw the footage for herself and was mortified.

Emily was terrified the clip could have been seen by colleagues and family. The couple didn't talk to each other for weeks.

So how does the industry - which exploits unsuspecting couples' intimate sexual acts for voyeuristic paying customers - work, and who is behind it?

One of the most prominent spy-cam porn traders I came across was an agent known as "AKA".

Posing as a consumer, I paid to access one of the livestreaming websites promoted by him - for a monthly fee of 450 Yuan ($65, £47).

Once logged in, I had the option to choose between five different filming feeds, each showing several hotel rooms - visible as soon as a guest triggered the electricity supply by inserting their key card. It was also possible to rewind livestreams from the start, and download archived clips.

On Telegram - banned in China but commonly used for illicit activity - AKA advertised these livestreams. One Telegram channel had as many as 10,000 members during the course of our investigation.

Libraries of his edited livestream clips are also available on Telegram for a flat fee. I could see more than 6,000 videos in the archive, dating back to 2017.

AKA's subscribers comment in Telegram's channel function, as they watch unsuspecting hotel guests - judging their appearance, gossiping about their conversations, and appraising their sexual performance.

They celebrate when a couple starts having sex - and complain if they switch the lights off, shrouding them in darkness. Women are regularly described as "sluts", "whores" and "bitches".

We managed to trace one of the spy-cams to a hotel room in Zhengzhou, central China, through piecing together several clues, from subscribers, social media users, and our own research.

Researchers on the ground were able to access the room, and found the camera - lens pointed at the bed - hidden in the wall ventilation unit and wired into the building's electricity supply.

A hidden-camera detector, widely sold online as a "must-have" for hotel guests, gave no warning they were being spied on.

The team disabled the secret camera and word spread quickly on Telegram.

"Zhonghua [name of the camera] got taken down!" one subscriber wrote on the main channel managed by AKA.

"It's such a shame; that room has the best sound quality!" AKA responded in the chat.

But the complaints turned to celebration when, within hours, AKA posted that a replacement camera in a different hotel had been activated.

"This is the speed of… [our livestreaming platform]," AKA told his subscribers. "Impressive right?"

During our 18-month investigation we identified about a dozen agents like AKA.

The exchanges they had with subscribers made it clear they were working for others higher up the supply chain, who they referred to as "camera owners". These people, the agents' comments suggested, arranged the spy-cam installation and managed the livestreaming platforms.

During our direct messaging with AKA, he accidentally shared a screenshot of a message from someone he said was a "camera owner", with the profile name "Brother Chun".

AKA quickly deleted the message, and refused to discuss it, but we managed to contact "Brother Chun" directly. Despite our evidence that he supplied the livestreaming website to AKA, Brother Chun claimed he was just another sales agent, although he did seem to acknowledge the supply chain extended beyond people like himself.

What is clear is there are significant sums of money to be made. Based on channel membership and subscription fees, the BBC estimates AKA alone has earned at least 163,200 Yuan ($22,000, £16,300) since last April. The average annual income in China last year was 43,377 Yuan ($6,200, £4,600) according to China's Bureau of Statistics.

There are strict rules on selling and using spy-cams in China, but we found it relatively easy to buy one in the country's largest electronics market in Huaqiangbei.

Accurate figures on how many people have been brought before the courts for spy-cam porn are more difficult to find. China's authorities have shared far fewer legal case details in recent years, but the cases we found ranged the length of China - from Jilin province in the north, to Guangdong in the far south.

Blue Li, from a Hong Kong-based NGO called RainLily - which helps victims remove explicit secretly-filmed footage from the internet - says demand is rising for her group's services, but the task is proving more difficult.

Telegram never responds to RainLily's requests for removal, she says, forcing them to contact group administrators - the very people selling or sharing spy-cam pornography - who have little incentive to respond.

"We believe tech companies share the huge responsibility in addressing these problems. Because these companies are not neutral platforms; their policies shape how the content would be spread," Li says.

The BBC itself told Telegram, via its report function, that AKA and Brother Chun - and the groups they managed - were sharing spy-cam porn via its platforms, but it did not respond or take any action.

Contacted again 10 days later, with the BBC's full investigation findings, Telegram told us: "Sharing non-consensual pornography is explicitly forbidden by Telegram's terms of service" and "it proactively moderates… and accepts reports [of inappropriate content] in order to remove millions of pieces of harmful content each day."

We formally set out our findings to Brother Chun and AKA that they were profiting from exploiting unsuspecting hotel guests. They did not reply, but hours later the Telegram accounts they used to advertise the content appeared to have been deleted. However the website that AKA sold me access to is still livestreaming hotel guests.

Eric and Emily remain traumatised by their experience. They always wear hats in public in case they are recognised and try to avoid staying in hotels. Eric no longer uses these Telegram channels to watch porn, he says, but still checks them occasionally - terrified the clip could resurface.