Concern mounted in Greece on Monday over the impact on its troubled banks and social security funds if the EU asks private lenders to take losses of at least 50 percent on their holdings of Greek bonds. The Athens stock exchange plunged by over five percent in early trading, with banks leading the fall after EU leaders at a summit in Brussels Sunday proposed the writedown or \"haircut\" as a solution to containing the Eurozone debt crisis. An earlier EU plan in July had only called for a 21-percent cut, with lenders exchanging their maturing Greek bonds with longer-term debt. A 50-percent writedown would mean that the country\'s main four banks -- National Bank, Eurobank, Alpha and Piraeus Bank -- would need a recapitalisation of 8.9 billion euros ($12.4 billion) to maintain a core capital ratio of 9.0 percent, analysts at Natixis bank said in a note. The four leading lenders had already written off 3.2 billion euros in the first quarter to enact the initial EU call for a 21-percent debt writedown. Speaking after the summit late on Sunday, Prime Minister George Papandreou pleaded for a \"viable\" writeoff for his country\'s debt, as Europe and the IMF struggled through negotiations on a cut of at least 50 percent. \"There is a willingness to face this challenge in order to get a viable solution,\" Papandreou said after day-long summit talks with fellow European Union and eurozone leaders in Brussels. He told a news conference after EU talks ended that Greece needed a \"viable solution to the Greek debt -- especially on the participation by private Greek banks,\" pension funds and insurance firms. The Greek leader told EU partners during the summit that the \"immediate debt reduction\" Athens would secure from a 50-percent writedown would only amount to 57.1 billion euros. A close advisor of Papandreou, former European Central Bank deputy head Lucas Papademos, had earlier warned that a 50-percent writeoff would only in reality amount to a 20-percent cut to Greece\'s debt mountain of over 350 billion euros ($487 billion). Papademos noted that Greek banks, pension funds and insurance companies in July held nearly 30 percent of central government debt -- and losses there would have to be offset by the cash-strapped Greek state. Another 30 percent held by the International Monetary Fund, European Central Bank and other official institutions could not be restructured for \"political and legal reasons,\" Papademos said. The Greek press meanwhile worried about the consequences of deeper writedown on Greek debt. \"Major haircut - we are playing with fire\" cautioned the pro-government daily Ta Nea. A cut of 60 percent \"would sustain Greek debt\", but there was, according to the paper, \"still the risk of both a default and that banks would be kept out of the market due to a lack of financing\". Right-wing Eleftheros Typos fumed: \"Nightmare haircut, a bomb for pension funds, bank nationalisation\". The left-wing Eleftherotypia newspaper said it feared that banks that resorted to funding from the European Financial Stability Facility, the European rescue fund, would move outside national supervision. The National Bank of Greece (BNG) and the small Attica Bank, which manages pension funds for engineers, would be particularly affected, the paper wrote. Greek pension funds are estimated to hold around eight billion euros in sovereign debt bonds and banks around 44 billion euros.