Facebook founder Mark Zuckerberg will testify against an upstate New York man accused of trying to cheat the billionaire out of half his take in the social media company, according to a federal prosecutor who spoke with Reuters.

Zuckerberg will be the key witness against Paul Ceglia, who has been charged with forging a 2003 contract with Zuckerberg that purportedly entitled him to half of Facebook.

"It's a witness that the government 100 percent knows it will be calling at trial," Assistant U.S. Attorney Christopher Frey said at a court hearing before U.S. District Judge Andrew Carter in New York federal court, according to Reuters.

Charges stem in part from a 2010 civil lawsuit Ceglia filed against Zuckerberg and Facebook in Buffalo, New York, claiming the two men signed a contract when Zuckerberg was a freshman at Harvard University that allowed Ceglia half of a planned social networking site.

Zuckerberg had done some programing work for Ceglia's company, StreetFax.com, and Facebook has confirmed the only valid contract between them related to that company.

Prosecutors in Manhattan charged Ceglia in 2012 with forging documents as part of the Buffalo litigation. Documents included the contract and supposed email correspondence with Zuckerberg.

A Buffalo federal judge dismissed Ceglia's lawsuit, determining the purported contract for an ownership stake in Facebook was in fact doctored, according to Reuters.

During a hearing on July 22, Carter denied a request from Ceglia's lawyers to authorize warrants for Zuckerberg's cell phones, bank records, and email accounts at Facebook from 2003 to 2004 as overly broad.

He also rejected their bid for Zuckerberg's Harvard email account and any other possible disciplinary records against him for unauthorized use of the school's computer system, according to Reuters.

The trial is set for Nov. 17.

See Now: OnePlus 6: How Different Will It Be From OnePlus 5?