Facebook, the fast-growing social networking site, is facing a potentially embarrassing lawsuit after a rival site claimed that the founder, Mark Zuckerberg, stole the idea for the network.

Mr Zuckerberg, 23, is accused of stealing the source code, design, and business plan for Facebook from ConnectU.com, a similar, university-based social network for which he worked briefly as a programmer four years ago.

ConnectU's owners, who were in the same year at Harvard as Mr Zuckerberg, claim that he stole not only "the basic idea" for Facebook from their site, but also ConnectU's specific code which, they say, was "proprietary and confidential."

The plaintiffs, Cameron and Tyler Winklevoss and their business partner Divya Narendra, are demanding that Facebook be shut down and that control of the site – and its profits – be handed over to them, according to a report on Portfolio.com.

The case comes at an inopportune time for Facebook, which last year reportedly turned down an offer as high as $1 billion from Yahoo! and is widely considered to be on the verge of listing on the stock market.

Accusations that the claimants are being opportunistic appear, however, to be misplaced. The lawsuit was first brought in August 2004, when Facebook had only 200,000 registered users, and it is only because of procedural delays that it has not been heard until now.

According to an article in the Stanford Daily three years ago, Mr Zuckerberg began working with the team from ConnectU.com in November 2003, doing coding, attending meetings and sending e-mails.

He then left the site, which at the time was called Harvard Connection, to develop his own ideas, and three months later launched Facebook, which the plaintiffs claim took away a valuable business opportunity.

Mr Zuckerberg has said he did six hours of coding for ConnectU on a voluntary basis, but said that he thought the site was a "personals page" and not a social networking site.

Mr Winklevoss disagreed, telling the paper: "It was clear to him what we wanted. He stalled us for months while he worked on his own idea, which he launched in February as an original idea."

In March this year, ConnectU's case was dismissed on a technicality, but because the legal merits were not ruled upon, the site's founders filed a new action, and a district court in Massachusetts is due to hear the case on July 25.

In a response to ConnectU's claims drafted by its lawyers, Orrick, Herrington & Sutcliff, Facebook reportedly said: "Only one of the students had an idea significant enough to build a great company. That person was Mark Zuckerberg."

Facebook, which now has about 39 million users worldwide, according to ComScore, has also countersued, accusing ConnectU of tort violations and unfair business practices, Portfolio.com reported.

A spokesman for Facebook was not immediately available for comment.

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