The summons came as the Italian Government denied that US authorities had told it in advance of the undercover operation against Hassan Mustafa Osama Nasr, also known as Abu Omar, who was suspected of links to international terrorism.

Abu Omar, the former imam of the main mosque in Milan, disappeared on February 17, 2003. At the time the Egyptian national was being investigated by magistrates in the city.

Italian prosecutors believe that he was abducted by the CIA as part of its programme of “extraordinary rendition”, in which suspected terrorists are transferred without court approval to third countries for interrogation.

The row became public last Thursday when a Milan judge signed arrest warrants for 13 alleged members of the CIA team which carried out the kidnapping.

“Our secret services were not aware of the operation,” Carlo Giovanardi, Minister for Relations with Parliament, told the Senate, specifically denying a report in The Washington Post that claimed the contrary.

“It was never brought to the attention of the Government or national institutions,” he added. “Consequently, it is not conceivable that any operation of this type was authorised or that Italian bodies were involved.”

Signor Giovanardi said that the Government would “take all possible measures to discover the facts” so that transgressions of national and international law could be laid bare.

He emphasised that this would be the case, “whoever committed them”.

According to Italian investigators, Abu Omar was forced into a van in broad daylight as he walked to the Milan mosque.

Either then, or some time later, he was taken to the US air base at Aviano in northern Italy, they believe.

Prosecutors say that they have found evidence of the cleric being taken to another US base, at Ramstein in Germany, before he finally ended up in prison in Cairo.

He was released in April last year and placed under house arrest, at which point he telephoned his wife in Italy and said that he had been tortured.

His present whereabouts are unclear, although some reports claim that he is back in prison in Cairo.

The case of Abu Omar is sensitive not only because of its implication that the CIA routinely ignores national sovereignty even in a country considered among its closest allies.

It also risks further straining US-Italian relations when Italians are just beginning to get over the emotional upheaval of the “friendly fire” killing of an Italian agent by American troops in Iraq this year.

Nicola Calipari was shot dead by soldiers at a road block as he was escorting the freed hostage Giuliana Sgrena to Baghdad airport on March 4. Signora Sgrena, a newspaper journalist, was injured.

In that case too, the question of who had informed whom about the mission was crucial. A joint US-Italian commission set up to establish what had happened ended up producing two separate reports, US and Italian members being unable to agree on responsibility.

Several members of the centre-left Opposition said that they refused to believe that the Government knew nothing of the CIA’s alleged hit.

Others, taking the Government’s word, accused the US of arrogance.

“The US would never have gone to grab somebody in Britain, France or Germany without informing authorities,” the opposition MP Antonio di Pietro said.

He added that Italy had become a “disposable” ally for Washington.

Mel Sembler, the US Ambassador, was away yesterday but was expected to respond to his summons from Signor Berlusconi as soon as he returns today.

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