Silvio Berlusconi, Italy's longest-serving postwar prime minister, has been handed his first definitive criminal conviction in more than 20 years of legal battles but the country's supreme court spared him the immediate prospect of being barred from public office.

In a long-anticipated ruling, the five judges of the court of cassation emerged from more than seven hours of deliberations to issue a verdict confirming a four-year jail term for the leader of the Freedom People party (PdL), a vital part of Italy's coalition government.

That sentence had already been cut to one year according to a 2006 amnesty, and, owing to Berlusconi's age – he will be 77 in September – it will be served through house arrest or community service.

It was enough, however, to place great pressure on the fragile government and prompt fury among his supporters.....