Rome, September 11 - Italian Premier Enrico Letta said Wednesday that political instability risks "costing the country billions" of euros, with his left-right executive in danger of collapsing in the fallout of the supreme court's decision to uphold a tax-fraud conviction against Silvio Berlusconi last month. Italy's borrowing costs have risen with Berlusconi's centre-right People of Freedom (PdL) threatening to pull its support from the government and sink it if its alliance partner, Letta's centre-left Democratic Party (PD), votes on a Senate panel for the three-time premier to be stripped of his seat in parliament. Letta said that Italy is no longer considered a threat to global financial stability after austerity measures and reforms passed by the emergency technocrat administration of his predecessor Mario Monti. But he warned that a government crisis could squander these efforts. "The great result of last week's G20 summit in St Petersburg is that Italy is no longer under special watch," Letta told the Senate. "We can ruin this in a moment by throwing away the stability conquered with hard work and we could easily return to conditions of very great difficulty".