Mastercard says it is ending the use of its cards on the pornography platform Pornhub after a review confirmed the presence of unlawful content.
A New York Times investigation accused the site of being "infested" with child-abuse and rape-related videos.
Pornhub, which has denied the claims, called Mastercard's actions "exceptionally disappointing".
Fellow payments giant Visa has suspended use of its cards on the site pending the outcome of its own review.
Mastercard launched its inquiry after Pulitzer Prize-winning journalist Nicholas Kristof named it in his New York Times article, saying he "didn't see why search engines, banks or credit-card companies" should "bolster" Pornhub.
"Our investigation over the past several days has confirmed violations of our standards prohibiting unlawful content on their site," a statement from Mastercard said. "We instructed the financial institutions that connect the site to our network to terminate acceptance."
The company said it was continuing to investigate possible illegal material on other websites.
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Pornhub's parent company MindGeek has previously said the claims made in the New York Times were "irresponsible and flagrantly untrue".
But the site has since removed its download function and said users must be verified to upload videos.
In response to Mastercard's decision, a Pornhub statement on Thursday said: "These actions are exceptionally disappointing, as they come just two days after Pornhub instituted the most far-reaching safeguards in user-generated platform history.
"Unverified users are now banned from uploading content - a policy no other platform has put in place, including Facebook, which reported 84 million instances of child sexual abuse material over the last three years. In comparison, the Internet Watch Foundation reported 118 incidents on Pornhub over the last three years."
It added: "This news is crushing for the hundreds of thousands of models who rely on our platform for their livelihoods."
Pornhub is free to use but users can pay £9.99 a month for higher-quality video streams and advert-free and exclusive content.
Its content is mostly uploaded by its own community and publicly viewable. However, the company said every video uploaded was reviewed by human moderators.
In its most recent annual review, Pornhub said it had had 42 billion site visitors in 2019 and more than 6.83 million videos had been uploaded, with a combined viewing time of 169 years. It did not say how many moderators it employed.
Kristof claimed searches for "under-age" videos yielded many results and while not all featured children, some appeared to.
Pornhub said it had "zero tolerance" for child sexual abuse and used a combination of tools from Google, YouTube and Microsoft to help it detect and remove illegal material.