MUMBAI: If the much-awaited Greek deal fails, the Reserve Bank will have to sell as much as USD 15 billion to defend the rupee at 65 levels, says a report.The embattled Greek government is set to present a compromise deal with the European Union and other global creditors to avoid a sovereign default later tonight."We see today's emergency summit of the heads of state (of European Union) as potentially the last chance to reach a deal. In our view, neither equities nor the rupee is pricing in any serious possibility of a breakdown of talks," Bank of America Merrill Lynch said in a report here today.In anticipation of a Greek deal, the markets have rallied today by 414 points, while the rupee ended barely unchanged at 63.51, down 0.06 per cent. After weeks of falling, the market had last week gained over 4 per cent. In last seven days, the index has gained 1,359.23 points, its longest winning run since February.The report expects RBI to sell USD15 billion to defend the rupee at 65 levels in case the Greek talks fail today.The report said Europe may seek to create a 'grey territory' situation, in which a default would not necessarily morph into a Grexit (or an exit of Greece from the EU and euro)."If Greece fails to pay the IMF on June 30 and the ECB on July 20, there is still a possibility it could remain in the euro regardless of the ELA funding being cut off," the report said.The report said if the dollar were to react sharply to the failure of Greek talks, the RBI could let the parallel rupee shock play out and enter only at 66 level.It expects RBI to pursue a three-pronged forex strategy -- at 60-62 level it will buy foreign exchange; at 63-64 level there would be a token resistance and at over 65 level it would sell USD 15 billion to defend 65 level.The report further said said it expects RBI will finally have to hike investment limit for overseas investors in government securities if there is a Greece deal comes through this week. Currently, FIIs debt investment limit is USD 30 billion.The report also said sovereign gold bond scheme will not really support the rupee unless the price risk is hedged.