* Vote is to lift immunity of former aide to Economy Minister
* Surprise not ruled out due to secret ballot
By Roberto Landucci
ROME, Sept 21 (Reuters) - A parliamentary vote on Thursday that could permit the arrest of a former top aide in the Italian government poses another potential embarrassment for Prime Minister Silvio Berlusconi,
Already embroiled in a prostitution scandal and hit by credit agency Standard and Poor's downgrade of Italy's credit rating this week, Berlusconi's capacity to resist growing calls to resign will be further tested by the vote.
Deputies are due to vote on a request by Naples magistrates to lift the parliamentary immunity of Marco Milanese, Economy Minister Giulio Tremonti's former right hand man, who has been accused of corruption and influence-peddling.
A vote in favour of lifting immunity would not have any direct effect on the government but it would be a severe embarrassment which would increase the uncertainty which already surrounds the fractious centre-right coalition.
Berlusconi's parliamentary majority has held up in numerous recent confidence votes, but a surprise cannot be ruled out, partly because Thursday's vote will be held by secret ballot, allowing any potential defectors to remain anonymous.
"It's obvious that with the secret ballot, other elements come out, like any discontent or feelings of unease," said Luigi Vitali, a deputy in the ruling PDL party, who highlighted the vulnerability of the centre-right coalition.
"It's not written anywhere that this legislature has to last out its full term. If we're going to keep on, it's not enough to just drift, everyone has to do their duty," he said.
Berlusconi has pledged to stay on until the end of his term in 2013 but he has been badly damaged by repeated scandal and the market turmoil which has pushed Italy to the brink of a full-blown financial crisis.
The centre-left opposition has made daily calls on the 74 year-old premier to quit and even Confindustria, Italy's main employers federation, says he should step down if he cannot implement swift reforms to revive Italy's stagnant economy.
However Fedele Confalonieri, chairman of Berlusconi's Mediaset media empire and one of his closest associates, dismissed questions from reporters about speculation he may step down with a disbelieving "Really?"
REFORMS
The difficulties facing the government come as Italy, the euro zone's third largest economy and one of its most sluggish, is fighting to avoid being dragged into a debt crisis that could destroy the single currency.
S&P's decision to cut Italy's credit rating was based on poor growth prospects but also on the damaging political uncertainty created by divisions in the coalition between Berlusconi's PDL party and the pro-devolution Northern League.
Tremonti has promised a decree with fresh economic reforms by the end of the month, with a focus on new infrastructure projects, although he has also promised the reforms will be passed "at zero cost" to avoid upsetting debt cutting plans.
A 59.8-billion-euro (${esc.dollar}82-billion) austerity package, passed in parliament this month with the aim of balancing the budget by 2013 has not succeeded in calming market worries reflected in a steady rise in the risk premium on Italian government bonds.
(${esc.dollar}1 = 0.730 Euros) (Writing By James Mackenzie; Editing by Michael Roddy)
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