Uber has sued an advertising firm, Fetch Media, over allegations that the British firm and its Japanese parent company, Dentsu, fraudulently billed Uber tens of millions of dollars for various fake online ads.

According to the lawsuit, which was filed Monday afternoon in federal court in San Francisco, Uber first realized that something was wrong when, earlier this year, the company began receiving complaints that its ads were appearing on Breitbart, a well-known conservative news website. Uber had specifically requested that its ads not appear on Breitbart at all.

However, when Uber looked into the matter, "the publisher-reported name of the websites and mobile applications where Uber advertisements supposedly appeared did not match the actual URL accessed. For example, one publisher retained by Fetch reported clicks on Uber ads as coming from placements such as 'Magic_Puzzles' and 'Snooker_Champion.'"

But, as it turned out, Uber found those ads were really coming from Breitbart. Such fraud is pervasive in the industry—one ad verification company, Adloox, estimated that ad fraud will earn advertisers $16.4 billion globally this year.

When Uber then cancelled its contract with Fetch in March 2017, the company expected to see a corresponding drop in installs, but no drop occurred.

"This indicated that a significant percentage of the installations believed to be attributable to advertising were in fact stolen organic installations," Uber's lawyers continued in the complaint. "In other words, these installations would have occurred regardless of advertising. Instead, networks or publishers in the Fetch Campaign fraudulently reported the last click attribution to claim attribution credit and were paid for the installation."

In a statement sent to media, James Connelly, Fetch's chief executive, said that his company was "shocked" at Uber's "unsubstantiated" claims that are seemingly meant as a diversion to "draw attention away from Uber's unprofessional behavior and failure to pay suppliers."

"Fetch takes ad fraud extremely seriously and has been working with clients and suppliers to minimize its impact within ad networks. It is unfortunate that Uber would misconstrue facts and use an industry-wide issue as a means of avoiding its contractual obligations," he continued. "We vigorously deny the allegations from Uber and will be responding robustly to ensure we set the record straight."