Ted Maher, 41, who said he was driven by his own "dark ideas", also admitted stabbing himself and inventing a cover story about two masked intruders armed with knives.
But Mr Maher, who had been working for Mr Safra since July, denied intending to kill his employer. He told police that he started the fire in Mr Safra's luxurious and heavily guarded apartment to "settle an account" with another member of the banker's staff.
Investigators see the billionaire's paranoia and obsession with security as partly responsible for his death and that of another nurse, Viviane Torrent. The pair locked themselves into a bathroom after Mr Maher raised his false alarm in the early hours of last Friday. Police sources say that Mr Safra, 67, refused to come out, even when his wife, Lily, told him on his mobile phone that the coast was clear.
Mrs Safra has angrily denied that version of events, but police sources are adamant that the banker refused to open the door to firefighters. When the rescuers finally broke down a security door, they found the billionaire and his 52-year-old nurse dead from inhaling fumes, which had come in through the bathroom's ventilation system.
Mr Maher, who is married with three children, will be charged with arson leading to the deaths of two people, which carries a possible life sentence in Monaco.
The chief prosecutor for the principality, Daniel Serdet, said yesterday that he accepted Mr Maher's claim that he had acted alone and had no murderous intentions. As Mr Safra's pounds 400-a-day private nurse, often alone with him day and night, he would have had "10,000 chances a day" to kill his employer if he wanted to.
Mr Maher's confession during his third police interrogation ends speculation that Mr Safra was assassinated by members of the Russian underworld. Banking sources had suggested a possible Moscow connection, because Mr Safra's New York bank blew the whistle last year on the embezzlement and money- laundering of hundreds of millions of dollars of international aid meant for Russia. The elucidation of Mr Safra's death, bizarre as the explanation is, should clear the way for the pounds 7bn sale of his New York and Luxembourg- based banking empire to the British banking giant HSBC. The US Federal Reserve was expected to give its opinion on the purchase of Mr Safra's flagship Republic New York Corporation last night.
Fire swept through Mr Safra's heavily protected two-storey apartment last Friday soon after the nurse appeared in the lobby with stab wounds to his leg. Mr Maher told security guards that he had been attacked by two hooded men with knives.
Police accepted his account at first but became suspicious when they saw tapes from security cameras inside the building and on the street outside, which showed no sign of the intruders. A bloodstained clasp knife found in the flat turned out to belong to Mr Maher.
The nurse confessed yesterday that he had stabbed himself and started the fire in a wastepaper bin. He told police: "It was my own dark ideas which led me to do this", to "settle an account" with an another member of the billionaire's medical staff. Mr Safra suffered from advanced Parkinson's disease and employed eight nurses and four doctors.
Mr Maher is thought to have quarrelled with one of the doctors recently, but his precise motivation for starting the fire and concocting the tale of an armed raid remains obscure. Monaco police are questioning Mr Safra's bodyguard, too, who was absent without leave on the night of his employer's death.
Mr Serdet, said that there seemed to be no financial motive for the nurse's behaviour. The nurse had told police that he was earning $600 a day and had "spoiled the most beautiful job he ever had".
Mr Serdet said that Mr Maher was "psychologically fragile and under the influence of medication".