Mr de Villepin was cleared in the case, but the state prosecutor appealed.
He had been accused of failing to stop the Clearstream corruption inquiry into Mr Sarkozy in 2004.
The BBC's Hugh Schofield reports from Paris that the outcome of the case could have a major impact on next year's presidential election.
In 2004, when both men were preparing for presidential bids, Mr Sarkozy's name appeared on a list of top politicians and businessmen who were wrongly linked to an illegal bank account in Luxembourg.
It was alleged those named on the list had received bribes from international arms sales.
The list was sent to people including Mr de Villepin, who was accused of failing to stop the conspiracy.'Technical trial'
Last year a judge cleared Mr de Villepin on all four counts of complicity to slander, to use forgeries, dealing in stolen property and breach of trust.
Several other defendants in the case were found guilty on various charges.
Our correspondent says that today the rivalry between Mr de Villepin and Mr Sarkozy has a new significance because of a presidential election now less than a year away.
Mr de Villepin has created his own political movement and is widely expected to announce a presidential bid.
With polls suggesting he could get 5% of the first-round vote, he could do real damage to Mr Sarkozy.
Our correspondent adds that complicating the picture are signs that the two men may have had something of a reconciliation in recent weeks - though how sincere that can be, given the mutual loathing that has so long existed between the two, must be a mater of doubt.
"We're going to move from a trial that was almost dramatic art to a more technical trial," Olivier Metzner, one of Mr de Villepin's lawyers, said ahead of the appeal case.
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