Just three months after centre-left Prime Minister Enrico Letta took office at the head of an uneasy coalition with Berlusconi's People of Freedom party (PDL), Italy is again mired in uncertainty.
The 76-year-old billionaire reacted angrily to the decision by the supreme court to reject a final appeal against his conviction, protesting his innocence and accusing magistrates of persecuting him since his entry into politics 20 years ago.
The ruling, confirming a sentence for tax fraud involving inflated invoices at his Mediaset broadcasting empire, was the first definitive sentence he had received after dozens of previous trials on charges ranging from tax to sex offences.
"No one can understand the real violence which has been directed against me," he said in a video message broadcast on Italian television after the verdict. "A genuine campaign of aggression that has no equal," he said.
Berlusconi is unlikely to have to serve any time in jail because of his age, and the supreme court ordered part of the original sentence, imposing a ban on holding political office, to be reviewed. But the ruling has dealt an unprecedented blow to the man who has dominated Italian politics for two decades.
"His conviction is like the fall of the Berlin Wall in 1989," crowed Beppe Grillo, leader of the anti-establishment 5-Star Movement and a ferocious critic of Berlusconi.
Berlusconi said he would continue his political activities under the "Forza Italia" (Go Italy!) name of his first party and press for a reform of the justice system, but he made no direct reference to the future of the coalition with Letta.
Impact on Letta government
Senior allies also reacted with bitterness, but said the ruling would not hit the coalition between Letta's centre-left Democratic Party and Berlusconi's People of Freedom.
"This sentence will not affect the Letta government, which was created to serve the country and which will continue to serve it as far as we are concerned," former Justice Minister Nitto Palma said after a meeting in Berlusconi's Rome residence.
But what might come over the next few months remains completely uncertain, with Letta struggling to contain increasing unhappiness in his own Democratic Party at the alliance with the scandal-plagued Berlusconi.
President Giorgio Napolitano, the man who would have to decide whether to call new elections if the ruling coalition fell apart, urged calm and said the country needed "serenity and cohesion." His comments were echoed by Letta.
"For the good of the country, it is necessary that, despite legitimate internal debate among political forces, a climate of calm and support for our institutions ensures that the interests of Italy prevail over party interests," he said in a statement.
But the ruling added another obstacle to Letta as he struggles to lead Italy out of its longest postwar recession, reform its stagnant economy and cut its mountainous public debt. More challenges may lie ahead.
As well as the tax fraud case, Berlusconi is also fighting a separate conviction for paying for sex with a minor, in the notorious "bunga bunga" prostitution case that tarnished his final months in office in 2011.
With the European Central Bank promising support if needed, investors have so far shown little concern.
That could change if prolonged political instability fuelled doubts about Italy's badly strained public finances and created the kind of pressure that brought down Berlusconi's last government as the eurozone crisis peaked two years ago.