Mr Gelli denies he was involved but has acknowledged that the financier, known as "God's banker" because of his links with the Vatican, was murdered. He said the killing was commissioned in Poland.
This is thought to be a reference to Calvi's alleged involvement in financing the Solidarity trade union movement at the request of the late Pope John Paul II, according to the sources quoted by La Repubblica newspaper.
Two Roman investigating magistrates, Judge Maria Monteleone and Judge Luca Tescaroli, sent Mr Gelli a judicial letter informing him that he is formally under investigation on charges of ordering the murder along with four other people - Flavio Carboni, a shadowy businessman with secret service contacts, his girlfriend Manuela Kleinsing, the Cosa Nostra boss Giuseppe Calo and an entrepreneur, Ernesto Dioatallevi. The four other suspects were indicted on murder charges in April and are to stand trial in October.
Investigators believe that Calvi was murdered as "punishment" for having used his position as head of the Banco Ambrosiano, then Italy's largest private bank, to seize large sums of money belonging to the Sicilian Mafia and to Mr Gelli.
The indictment also says that the five ordered Calvi's murder to prevent the banker "from using blackmail power against his political and institutional sponsors from the world of Masonry, belonging to the P2 lodge, or to the Institute for Religious Works [the Vatican Bank] with whom he had managed investments and financing with conspicuous sums of money, some of it coming from Cosa Nostra and public agencies".
Nearly 1,000 prominent public figures including businessmen such as the current Prime Minister Silvio Berlusconi, senior army and police officers, politicians and civil servants belonged to Mr Gelli's Propaganda Due (P2) clandestine Masonic lodge, dissolved in 1981 for plotting to establish an authoritarian regime.
When interrogated by magistrates in the presence of his lawyer on 4 July, Mr Gelli strongly denied having ordered the murder of Calvi, the sources said.
The former grand master said he had known Calvi since 1975 when he was introduced to him by Umberto Ortolani, another leading P2 member, but that he had few dealings with the Banco Ambrosiano, the collapse of which in 1982 sent Calvi fleeing to London.
The only dealing he had was in 1981 when he loaned $10m to the bank's Nassau subsidiary in the Bahamas, which was repaid to him one month later, he said.
"Calvi's death was made to look like suicide," he told the magistrates. Mr Gelli said the murder was related to Calvi's dealings with the Vatican Bank, which has always denied any moral responsibility in the Ambrosiano affair. "One evening I was at dinner with Calvi. He was angry, black in the face. He told me that the next day he had to go and see 'the most Holy one' in the Vatican to get $80m that he had to pay for bills relating to Poland and that if he did not get the money everything would blow up," Mr Gelli was quoted as saying in La Repubblica.
"This happened between 1979 and 1980, and that is why I said that to find Calvi's assassins one ought to have investigated in Poland," Mr Gelli was quoted as telling the magistrates.
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