Silvio Berlusconi, who last week took office for the third time as the Italian Prime Minister, can be called to testify in a trial of American and Italian agents accused of kidnapping a terrorism suspect in Milan, a judge has ruled.

The move would make Mr Berlusconi the first serving head of government to testify in criminal proceedings over "extraordinary renditions", the secret transfer of terrorism suspects by the United States to third countries. If summoned he is obliged to appear in court.

The agents are accused of kidnapping an Egyptian-born imam at a Milan mosque and flying him to Egypt to be interrogated and allegedly tortured. Mr Berlusconi is not accused of a criminal offence, and would appear as a witness on the question of whether information in the case is covered by regulations on "state secrets".

Mr Berlusconi, who won a commanding majority in last month's election and was confirmed in office by a confidence vote in the Lower House of Parliament this morning, was Prime Minister when the kidnapping took place in 2003.

Judge Oscar Magi ruled that Romano Prodi, the Prime Minister from 1996 to 1998 and again from 2006 until his election defeat, could also be called to testify on the same question.

Twenty-six Americans, all but one described by the prosecution as CIA agents, are being tried in absentia. The United States has refused to extradite them. Prosecutors say a CIA-led team kidnapped Hassan Mustafa Osama Nasr, known as Abu Omar, on a Milan street and secretly flew him to Egypt.

Mr Nasr, who now lives in Alexandria,Egypt, has claimed that he he was held without charge and tortured before being released last year. He says he suffers from heart and kidney problems.

Italian prosecutors admit that he was under investigation for alleged terrorist activities at the time of his kidnapping, but they say that the alleged CIA kidnapping undermined their inquiry and was a breach of Italian sovereignty.

Mr Berlusconi, a staunch ally of the Bush administration, has in the past denied that he either approved or was informed of the CIA operation, and has defended the Italian intelligence service chiefs against charges that they secretly colluded in it.

Niccolo Pollari, former head of Italian intelligence, who is accused of collusion, maintains that "classified documents" prove he was not involved. His lawyers said that Mr Pollari wanted Mr Berlusconi and Mr Prodi to acknowledge this in court by admitting the documents existed and describing their contents. However the Constitutional Court has been asked to rule on whether prosecutors in Mr Nasr's case have violated state secrecy regulations by bringing the case. A Constitutional Court ruling that information in the case is a "secret of state" could lead to the Milan trial being abandoned.

Today, Nabila Ghali, Mr Nasr's wife, gave evidence at the trial, saying she that had got to know the imam while attending his lessons on Islam in Milan and married him in 2001. Her face covered by a veil, she said that on the day of the kidnapping, on February 17 2003, she became alarmed when her husband failed to answer his mobile phone and officials at the Milan mosque said he had not been seen.

She said a local woman had later described seeing Mr Nasr "talking to someone in Italian" on the street before being "bundled into a van" by two men. She did not find out he was in Egypt until over a year later.

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