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Facebook founder Mark Zuckerberg has said that an alleged contract and e-mails that a New York man claims entitle him to a 50% stake in the social networking site are "forgeries".

Paul Ceglia says that Mr Zuckerberg signed a contract in 2003 which gave Mr Ceglia half-ownership of Facebook.

In a court filing, Facebook and Mr Zuckerberg demand that Mr Ceglia turn over the alleged contract and e-mails.

Lawyers representing Mr Ceglia disputed Facebook's claims.

"Those so-called expert opinions have been provided without examining the actual contract which is at issue in the case," said Dennis C Vacco, who is one of Mr Ceglia's attorneys.

In a filing made at the US District Court in Buffalo, Mr Zuckerberg said he provided web development services in 2003 for StreetFax, a business Mr Ceglia was trying to start at the time.

He said he signed a contract drafted by Mr Ceglia, a wood pellet salesman, which referred only to the work he did for StreetFax.

"Zuckerberg and Ceglia never discussed Facebook and they never signed a contract concerning Facebook," the filing said.

"The contract is a cut-and-paste job, the e-mails are complete fabrications, and this entire lawsuit is a fraud."

Facebook and Mr Zuckerberg hope to use forensic testing to show that the documents are fakes.

Facebook is privately owned but estimates of its worth range between $50bn (£30bn) and $76.4bn (£46.7bn).

Twins Cameron and Tyler Winklevoss also claimed that Mr Zuckerberg stole their website idea while they were all students at Harvard.

In 2008 they reached a settlement which gave them $20m in cash and $45m of stock valued at $36 a share.

They have since unsuccessfully tried to reopen their case against Facebook, claiming that the company concealed information and they should have received more shares.