LISBON (Reuters) - A former CIA agent will be extradited to Italy for her involvement in the kidnapping of an Egyptian cleric under the U.S. "extraordinary rendition" program, after a last appeal to Portuguese authorities failed, she said on Wednesday.

Sabrina De Sousa, a joint U.S.-Portuguese citizen, was one of 26 people convicted in absentia on charges of snatching Hassan Mustafa Osama Nasr from a street in Milan in 2003 and taking him to be questioned in Egypt.

The case focused attention at the time on the treatment of suspects moved around the world for interrogation in the wake of the Sept. 11, 2001, attacks and strained relations between Washington and Rome.

De Sousa was arrested in Portugal in October at the request of Italian prosecutors, who want her to serve a six-year sentence. De Sousa lodged an appeal against the extradition in January but Portugal's constitutional court has now rejected it.

"Yes, it (the rejection of the appeal) was confirmed about 10 days ago," De Sousa said in a an email response to Reuters. "The (extradition) process started last night. I have no idea of the date because Portugal (Interpol) have to contact Italy to agree on a date."

She said she had no idea if she would now have to serve the prison sentence in Italy or if she would be given a new trial to be able to counter the charges, as she has requested.

De Sousa has sent a letter to the Pope appealing for his intervention.

"Italy has guaranteed Portugal that I will be notified of my sentence and have the opportunity to counter the charges against me," De Sousa wrote in the letter, which she provided to Reuters. "Whether that happens, remains to be seen."

Some of the people convicted in Italy for being involved in the rendition of the Egyptian cleric have been pardoned.

(Reporting By Axel Bugge and Shrikesh Laxmidas; Editing by Richard Balmforth)