"This lawsuit is a brazen and outrageous fraud on the court," Facebook said in its legal response in the US District Court in New York. "Plaintiff is an inveterate scam artist whose misconduct extends across decades and borders."
The plaintiff, Paul Ceglia, claimed that he had funded Facebook founder Mark Zuckerberg's work to create the basis of what would become the world's most popular social networking site, which is now valued well in excess of 50 billion dollars.
In his lawsuit, Ceglia submitted what he said was a legal agreement signed between him and Zuckerberg at the time entitling him to 50 per cent of Facebook.
But Facebook lawyers accused him of forging the document using a different agreement under which Zuckerberg did development work for Ceglia for a website called Streetfax.com.
"To be clear, Zuckerberg did not sign the purported agreement, which is a 'cut-and-paste' job fraudulently manufactured by plaintiff for this lawsuit," Facebook said.