The former president of the European Commission, Jose Manuel Barroso, told CNBC Wednesday that there was a risk Greece would be unable to come to an agreement with its euro zone creditors.  He said this could increase the risk of the country defaulting on its debt obligations or having to leave the euro zone—something he called a "taboo" scenario, which could set a dangerous precedent for the single currency bloc.  Read MoreRisk of a 'Grexit' is higher today: Barroso  While political analysts at Nomura, Alastair Newton and Frida Wallnor, put the probability of a Grexit at 40 percent in a note entitled "Game over for Greece?" published Wednesday, they warned that their prediction was "coupled with a sense that (the probability) is still rising with no clear sign of a reversal. "  "The main reason is what we see as the continuing propensity for rhetoric not just to dominate, but to overwhelm, substance in the exchanges between Greece and its euro zone partners," they wrote. "Indeed, we would go further and suggest that Greece may be closer to an exit today than it has been at any time since the start of the euro zone crisis."  Despite growing fears of a euro zone exit, some euro zone officials have refused to countenance such a scenario, which could bring with it significant upheaval and potentially disastrous consequences for the euro zone.  Not only could a default and Grexit prompt capital controls to prevent bank runs, international financial isolation and the introduction of a new currency in Greece, it could threaten the future of the 19-country single currency bloc.  Read MoreWhat would a Greek default look like?  Knowing that any such talk could spark international panic over Greece and the intergrity of the euro zone and its currency, the European Central Bank's President Mario Draghi dismissed fears of a Greek default Wednesday, saying he was not ready to even "contemplate" such a scenario.  Officials in the U.S. have openly warned over the risks posed by Greece, however. Greek Finance Minister, Yanis Varoufakis, is due to meet U.S. President Barack Obama on Thursday, and U.S. Treasury Secretary Jacob Lew on Friday (along with the ECB's Draghi and IMF officials).  Ahead of the meeting with his Greek counterpart, Lew said Wednesday that he would press Varoufakis to "engage proactively" with European officials to resolve the standoff over emergency financing, Dow Jones reported Wednesday.
A U.K. bookmaker has stopped taking bets on Greece leaving the euro zone, saying it is increasingly likely the country could depart "very shortly."