The Italian news agency Ansa said his provisional release had been ordered because he had significantly changed his contention that he had been forced to pay bribes to political parties to win contracts. Mr De Benedetti had been collaborating and had given investigators new leads, it quoted judicial sources as saying.
Mr De Benedetti, the most prominent and richest industrialist yet to be detained over alleged corruption, had been sought since an arrest warrant was issued for him on Saturday. He presented himself at a Milan carabinieri station at 4.30am yesterday and was driven, accompanied by his lawyer Marco de Luca and a plainclothes escort, to the former convent by the Tiber.
He was taken to a side entrance but was nevertheless spotted by the crowd of waiting reporters and cameramen. Visibly upset, he declined to make any comment as he was driven in to face the humiliating ritual to which all who enter are subjected - fingerprinting, photographs, removal of valuables and the walk to the cells.
He was later questioned by the supervisory magistrate who signed his arrest warrant, Augusta Iannini, and by the investigating magistrate, Maria Cordova, both of whom he greeted by kissing their hands.
The magistrates were probably not immediately impressed by Mr De Benedetti's insistence that he was not the corrupter of politicians - to whom he is alleged to have given large bribes to get big government contracts - but the corrupted, since the money had, he claimed, been extorted, not foisted upon them.
The Milan magistrates, to whom he made a spontaneous confession in May, appeared to have accepted his argument. But Ms Cordova, who has since taken over the case, maintains that more elements have come to light since then.
The arrest warrant stated that Mr De Benedetti had left gaps in his story and spoken in such general terms as to hamper the search for truth. If allowed to remain free, he might repeat his 'anti-juridical behaviour', particularly in view of his position in Italy's 'power structure', it said.
The industrialist yesterday received strong public support from Monsignor Luigi Bettazzi, the Bishop of Ivrea, the Olivetti town in Piedmont, who published an article raising the possibility that Mr De Benedetti had been the victim of machinations by his enemies.
The Ivrea town council, like Olivetti trade-union leaders the previous day, expressed alarm about his arrest and called for a speedy end to the case. The company is badly hit by the recession and has made losses three years running.
Shares in Olivetti fell slightly on the Milan stock exchange yesterday. Analysts said investors were already accustomed to such cases and would only become alarmed if Mr De Benedetti were jailed, Reuters reported. 'Unless the magistrates have come up with something really quite extraordinary we're not going to be shocked,' one was quoted as saying.