Rome, September 10 - Italian Premier Enrico Letta on Tuesday cancelled an appearance at the political "Summer School" for students organized by ex-premier Silvio Berlusconi's People of Freedom (PdL) party. The premier, who hails from the opposing center-left Democratic Party (PD), did not say why he would not attend the event held in the countryside outside Rome. The PdL, which shares government power with the PD in an unprecedented alliance, has vowed to pull its support and topple Letta's executive if a Senate panel votes later Tuesday to reject petitions it has presented to forestall a decision on whether Berlusconi should be stripped of his Senate seat after he was definitively convicted last month for tax fraud at his Mediaset media empire. The PdL says the six-year ban is unconstitutional, since it applies to a 2012 law that went into effect after the three-time premier committed his crimes. The PD says the law must be implemented since it became active before Berlusconi was convicted. The PdL pulling its support from the government may force snap elections - the second this year after an inconclusive February vote led to two months of political deadlock, put to an end by the formation of the current left-right government.