A US federal appeals court has ruled that former Harvard University friends of Facebook founder Mark Zuckerberg cannot undo their settlement over creation of the social networking site.

The 9th US Circuit Court of Appeals in San Francisco said Tyler and Cameron Winklevoss were savvy enough to understand what they were agreeing to when they signed the agreement in 2008.

The deal called for a 20 million US dollars cash payment and partial ownership of Facebook.

A third classmate, Divya Narendra, was part of the settlement with the twins but did not pursue the second lawsuit seeking to undo the agreement.

The latest ruling upholds a lower court decision enforcing the settlement during the six years of litigation which grew so contentious that the dispute was dramatised in the Oscar-nominated film The Social Network.

The settlement is now worth more than 160 million dollars because of Facebook's increased valuation.

The twins had alleged they were misled about Facebook's value when they agreed to settle their claim that Mr Zuckerberg stole their idea to launch Facebook.

"At some point, litigation must come to an end," chief justice Alex Kozinski wrote for the unanimous three-judge panel. "That point has now been reached."

The twins alleged they were misled into believing the company was worth 35.90 dollars a share because of an investment by Microsoft. But they argued the company was later valued the at 8.88 dollars for tax purposes. They said they would have demanded more stock in the company based on the lower valuation.

Mr Kozinski said the twins were "sophisticated parties" when they agreed to the settlement during a mediation meeting.

"They brought half a dozen lawyers to the mediation," he wrote.

Facebook said it was pleased by the ruling.

Lawyers for the Winklevoss twins said they are reviewing the decision and have not decided on their next step. The twins could ask the Supreme Court to consider the case.