* Court rules no "legitimate impediment" to fraud hearing
* Berlusconi risks four-year jail sentence, ban on office
* Ruling opens way to final verdict later in the year
* Berlusconi pledges continued support to Letta government
Italy's constitutional court on Wednesday rejected a bid by Silvio Berlusconi to block a tax fraud conviction from October, clearing the way for a final verdict this year and deepening the uncertainty facing Prime Minister Enrico Letta.
Berlusconi issued a statement immediately after the ruling, saying the decision was contrary to good sense and legal precedent but would not affect his support for Letta's left-right coalition government.
The 76-year-old media tycoon is not in government but Letta depends on his People of Freedom (PDL) party to survive after deadlocked elections in February which left his own centre-left Democratic Party (PD) short of a viable majority in parliament.
Despite Berlusconi's assurance of continued support, there has been wide speculation that the ruling will increase tensions in the already fragile coalition. Asked if the ruling would impact the government, Regional Affairs Minister Graziano Delrio said: "We will see".
Berlusconi's lawyers had challenged a ruling in Milan last year that sentenced him to four years in prison with a five year ban on holding public office after it found him complicit in tax fraud at his Mediaset broadcasting empire.
They argued that the case was flawed because judges had refused to allow a delay when Berlusconi, who was prime minister at the time, claimed he was unable to attend a hearing in March 2010 because of a cabinet meeting on the same day.
The constitutional court ruled that it was up to the judges of the original case to rule on whether Berlusconi's claim that the cabinet meeting constituted a so-called "legitimate impediment" to the hearing.
It also noted that he had been given several opportunities to agree an acceptable court date and had called the cabinet meeting despite previously declaring his availability for the hearing on the same day.
A definitive verdict in the case, in which Berlusconi was found to be complicit in inflating the price of broadcast rights contracts to skim off cash for illegal slush funds, rests on the outcome of a final appeal expected later this year.
Lawyers consider that he is unlikely to serve any actual jail time, whatever the result of the appeal, but political fallout from a final conviction would be unpredictable, and he would not be able to return to government for five years.
Had the constitutional court accepted Berlusconi's argument, the trial probably would have run into Italy's statute of limitations and prevented a final verdict ever being reached before the case expires next year.
The ruling is only one of a series of potentially explosive cases on accusations ranging from paying for sex with a minor to bribing political opponents which could seriously test the stability of the government.
The most serious is the verdict expected on Monday on charges that Berlusconi paid for sex with former teenaged nightclub dancer Karima El Mahroug, better known under her stage name "Ruby the Heartstealer" when she was under 18.