(Reuters) - Three U.S. lawmakers and two in Britain called for tougher safeguards against online sexual exploitation after a Reuters investigation this week identified more than 140 police complaints of nonconsensual pornography on OnlyFans, the popular adults-only website.

"It is absolutely unconscionable," U.S. Representative Ann Wagner, a Missouri Republican, said in a statement about the abuses alleged in the complaints.

"These findings confirm what my office has known for years: Americans are being sexually exploited on OnlyFans," said Wagner, who sponsored a federal law passed in 2018 that allows victims to sue websites that knowingly host abusive commercial sex activity. "Congress and federal law enforcement must do more."

Through public records requests to the largest U.S. law enforcement agencies and a review of court cases, Reuters identified 128 cases in which adults complained that sexual content featuring them had been posted on OnlyFans without their consent between January 2019 and November 2023. A Florida woman, alleging a video of her rape was posted and sold on OnlyFans, filed a lawsuit in November 2022 against the platform under federal laws including Wagner's.

In the UK, where OnlyFans is based, Reuters documented 18 complaints of nonconsensual pornography appearing on the site.

"Social media platforms have become a safe harbor for predators," U.S. Senator Dick Durbin, an Illinois Democrat, said in a statement to Reuters. "Rape victims - including children - are not only violated at the time of their assault, but they are victimized over and over again with the rapid spread of their abuse material online. Even worse, the platforms profit from this activity. That's unacceptable."

U.S. Representative Jennifer Wexton, a Virginia Democrat, called for greater accountability for perpetrators to ensure "we are protecting the rights and lives of all victims and survivors."

OnlyFans says on its website that it is building "the safest social media platform in the world." Its CEO, Keily Blair, has said the company reviews 100% of content using human moderators and artificial intelligence tools.

In a statement responding to Reuters findings, an OnlyFans spokesperson said that "in the few examples where bad actors have misused our platform," OnlyFans "removed the content swiftly, banned the user and actively supported investigations and prosecutions." She added that OnlyFans tightened its consent verification procedures in late 2022.

Reuters found more than a dozen complaints of nonconsensual porn that were filed after that.

In a court filing, OnlyFans' U.S. subsidiary, Fenix Internet, has said it will seek to have the Florida woman's sex-trafficking case dismissed, citing a federal law that gives websites immunity from suits based on their users' posts.

UK-based OnlyFans is one of 10 companies, including social media platforms and a porn site, that partner with stopNCII.org, a British project designed to prevent the sharing of non-consensual intimate images (NCII) online.

David Wright, CEO of South West Grid for Learning, the British charity that runs stopNCII.org, did not address Reuters findings about OnlyFans, but said: "We encourage all platforms to make the same steps as OnlyFans to prevent NCII and work towards the eradication of intimate image abuse."

Damian Collins, a British member of parliament, said the Reuters investigation "shows there are clearly gaps in the systems being used by OnlyFans, and Ofcom, the UK (communications) regulator, should challenge the company based on this evidence."

Collins chaired a parliamentary committee on Britain's draft Online Safety Bill, which became law in October 2023. The law requires social media companies to prevent and quickly remove illegal content, including nonconsensual porn, and to stop children from accessing harmful content or pornography.

Ofcom can fine companies that don't comply up to 18 million pounds ($23 million) or 10% of their global revenue, whichever is greater. Enforcement of the law is proceeding in phases.

OnlyFans has said it does not know how much adult content is on its site. It also features sports, music and other non-sexual content.

"I am concerned that if OnlyFans cannot say how much adult content is on its platform, or how many accounts are dedicated to sharing user generated pornography, then it does not have sufficiently robust monitoring systems," Collins said in a statement. "This cannot just be left to uploading tools and AI, there also needs to be effective human oversight of the platform's policies."

A spokesperson for Ofcom told Reuters that tackling sexual abuse and other illegal online harms will be its priority under the Online Safety Act. The regulator is "already actively engaging with many services about the robustness of their protection measures, including OnlyFans, under the existing video-sharing platform regulations," the spokesperson said.

"We've seen some improvements as a result of this engagement, but there's more to do."

In a January report on video-sharing platforms, Ofcom said OnlyFans had introduced measures to verify the age of its British users and made it easier for them to report child sex abuse material.

In a post on X, James Bethell, a Conservative member of the House of Lords and campaigner on online harms, said he was "skeptical" that OnlyFans offers "a secure platform for sharing legitimate porn content."

He expressed concern about the site's distinctive subscription model, in which individual creators' content is hidden behind a paywall.

"OnlyFans has great PR. But their paywall prevents law enforcement from searching for illegal content," he wrote.

(So reported from Washington D.C. and Marshall from London. Jason Szep and Rosa Furneau contributed reporting. Editing by Julie Marquis)