Citigroup Inc. said there's now a 90 percent chance that Greece will leave the euro in the next 12 to 18 months, with prolonged economic weakness and spillover for the currency bloc.

In an analyst note, Citigroup updated its forecast for a Greek exit from the 17-nation currency union from a previous estimate of 50 percent to 75 percent, and said it would most likely happen in the next two to three quarters. Specifically, the bank assumes a Greece exit would occur on Jan. 1, 2013, while saying that is not a forecast of a precise date.

Greece's so-called troika of international creditors, the European Central Bank, the European Commission and the International Monetary Fund, are in Athens this week amid doubt the country will meet its bailout targets and reluctance among Germany and other euro-area states to put up more funds should Greece fail to do so.