* Court issues reasons for Berlusconi tax fraud conviction
* Throws spotlight back on ex-PM's legal woes
* Berlusconi says Italians won't permit his ousting from Senate
Former Prime Minister Silvio Berlusconi was the mastermind behind a complex system of tax evasion by his holding company Fininvest, Italy's highest appeals court said on Thursday.
The Court of Cassation, which published the reasons for its decision earlier this month to uphold Berlusconi's conviction for tax fraud, pulled no punches in a 208-page document which dismissed all the media tycoon's arguments in his defence.
The court's ruling on Aug. 1 threw Italian politics into disarray when it handed Berlusconi his first definitive jail sentence, of four years commuted to one.
Under a law passed by Mario Monti's government last year that conviction means Berlusconi, who sits in the Senate, also faces the risk of expulsion from parliament.
Berlusconi was "directly responsible for the masterminding, creation and development" of a system set up "with the aim of evading Italian taxes," the Court of Cassation said.
The publication of the document by the panel of five judges throws an unwelcome spotlight back on Berlusconi's legal woes, which threaten to upset Enrico Letta's unstable government.
The government has bickered continuously since taking office in April but won a respite on Wednesday when it reached agreement over reform of a housing tax that had been a constant source of division between the left-right coalition of former rivals.
Letta faces a bigger hurdle on Sept. 9 when a Senate panel meets to begin deciding whether Berlusconi should be thrown out of parliament. Berlusconi's allies have threatened to bring down the government if Letta's Democratic Party (PD) vote to oust him, as they have said they will.
Berlusconi said in a television interview with his Italia Uno channel on Thursday that his tax fraud sentence was "based on nothing," and that millions of Italians "will not permit" his expulsion.
Under the law passed last year, deputies or Senators convicted of criminal offences can be expelled. Berlusconi's lawyers have argued the law cannot apply to him as it was passed after the offences for which he was convicted.
Berlusconi, who insists he is the victim of a politically motivated court, is showing all his renowned fighting spirit.
He plans to appeal to Italy's constitutional court against the tax fraud verdict. His People of Freedom party (PDL) says the Senate should wait for that ruling before deciding whether he must leave parliament.
However, even if the constitutional court agrees to hear his case, it may take years to reach a decision, and Berlusconi's opponents say he is just playing for time.
Berlusconi has also appealed to the European Court of Human Rights in Strasbourg.
In addition to the tax fraud case, he is fighting a 7-year jail sentence issued by a Milan court this year for paying for sex with an under-aged prostitute and abusing his office to cover it up.