A judge ruled there were no constitutional grounds to reverse a decision by a lower court and the Supreme Court to send Sabrina de Sousa, who was born in India and holds both US and Portuguese passports, back to Italy, the official told the AP.
He spoke on condition of anonymity in accordance with the Constitutional Court's rules.
De Sousa was among 26 Americans convicted in absentia for the 2003 kidnapping in Milan of terror suspect Osama Moustafa Hassan Nasr. She was arrested last October at Lisbon Airport on a European warrant.
De Sousa's Portuguese lawyer, Manuel Magalhaes e Silva, was not immediately available for comment. Lisbon court officials could not be contacted to provide the date when an extradition might take place.
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Her Italian lawyer has said he is hopeful of obtaining clemency from Italy's head of state. President Sergio Mattarella has granted clemency to other defendants convicted in the case.
The extraordinary rendition program was part of the anti-terrorism strategy of the U.S. Administration following the September 11, 2001, attacks. President Barack Obama ended the program years later.
De Sousa was on her way to visit her elderly mother in India with a roundtrip ticket when she was detained.