Rome, June 19 - Italy's underground economy has become so large that it is second only to Greece in international rankings, a senior audit official said Wednesday. The size of the economy that exists outside the legal tax system is "significant" - up to 18% of national GDP, said Luigi Giampaolino, president of the national Audit Court. Put another way, avoidance of such taxes as the VAT meant that as much as 50 billion euros slipped away from the taxman in 2011, he said. "That places our country in second place in the international rankings (of tax avoidance) led by Greece," Giampaolino told a parliamentary hearing. There is a "high propensity to avoid declaring taxes".