Abu Omar was suspected of recruiting Islamist fighters
Italian PM Silvio Berlusconi will be called as a witness in a trial over the alleged CIA kidnap of a terror suspect.
Twenty-six Americans and six Italians are accused of kidnapping a Muslim cleric from Italy and sending him to Egypt, where he claims he was tortured.
A judge in Milan ruled that Mr Berlusconi, who faces no charges in the trial, could be called to testify.
Former spy chief Nicolo Pollari says testimony from ex-heads of government may prove he was against the practice.
Mr Berlusconi is considered a key witness as he was prime minister when prosecutors allege that Egyptian cleric Hassan Mustafa Osama Nasr was snatched from a street in Milan, in February 2003.
Italian prosecutors say he was taken as part of a series of extraordinary renditions carried out by the CIA - when terror suspects were moved between countries without any public legal process.
Judge Oscar Magi ruled that former Prime Minister Romano Prodi can also be called as a witness during the trial.
The US agents and military personnel will be tried in absentia.
Italian prosecutors say Mr Nasr was taken to US bases in Italy and Germany before being taken to the Egyptian capital of Cairo.
Mr Nasr says he was tortured during his four-year imprisonment in Cairo.
At the time of his arrest he was suspected of recruiting fighters for Islamic groups but had not been charged.
He was released by Egypt in February 2007.