The ruling against Mr. Lindell was earlier reported by The Washington Post.

Mr. Zeidman, 63, who is from Las Vegas, filed the arbitration claim against Mr. Lindell in November 2021 after the contest’s organizers rejected his findings. The claim was filed in Minnesota, Mr. Lindell’s home state.

The arbitrators ordered him to pay Mr. Zeidman within 30 days.

Mr. Lindell, who has spent millions of dollars on partisan reviews of voting data and efforts to bolster election skeptics across the country, vowed in an interview to challenge the panel’s ruling.

“This is disgusting,” he said. He questioned Mr. Zeidman’s credentials and mused about how he had been granted admission to the symposium.

Mr. Zeidman, who described himself as a “well-known” pioneer in the field of software forensics, said that he used his connections in the Trump world to obtain an invitation to Mr. Lindell’s symposium.

“Friends of mine said, ‘You should go because you might win $5 million,’” he said.

When conference organizers gave Mr. Zeidman and other attendees data to dissect, he said that he expected it might take weeks to analyze. But once he started going through the files, he said he quickly concluded that the data was bogus. He presented his findings to Mr. Lindell’s representatives in a 15-page report.