By Sumanta Dey
July 8 (Reuters) - Greece may be about to become the first country to step through the exit door and leave the euro zone, according to a Reuters poll that put the chances of the monetary union breaking up at more than 50 percent for the first time.
Euro zone members have given Greece a final deadline of Sunday to come up with a proposal for sweeping reforms in return for loans that will keep the country from crashing out of Europe's currency bloc and into economic ruin.
But months of negotiations between Athens and its creditors has so far yielded nothing and Greece became the first developed economy to default on a debt payment to the International Monetary Fund last month.
Trust has also eroded between both sides after Prime Minister Alexis Tsipras unexpectedly called a referendum in which six out of 10 Greeks voted to reject further austerity.
"It's gone too far in the sense that there is absolutely no trust whatsoever any longer," said Christel Aranda-Hassel, senior European economist at Credit Suisse, referring to the long-drawn-out negotiations.
"Ultimately, I think the Greeks have overplayed their hand."
Fifty-seven economists polled on Wednesday, as Tsipras pleaded in the European Parliament for a fair deal for his country, gave a median 55 percent chance of Greece leaving the euro zone.
That is the first time the median probability has shown Greece is more likely than not to leave the euro in many years of Reuters polls asking the same question.
While the results show economists have begun warming up to the possibility of "Grexit" -- for many large banks it is now their base case -- it also reflects a glimmer of hope a deal could be clinched just in time.
Financial markets also appear to have either held on to that hope or have dismissed a possible Greek exit as a non-event.
The euro has barely weakened this week while bond yields for peripheral countries such as Portugal, Spain and Ireland have even dipped a little.
"It's (a deal) still not impossible especially if the Greek government comes up with a new proposal and does not insist on a nominal haircut on the debt but accepts lengthening of the maturities," said Johannes Mayr, economist at BayernLB.
Tsipras has so far insisted on debt relief, saying years of austerity has damaged Greece's economy and heaped misery on people without any rewards.
While IMF Chief Christine Lagarde has hinted some sort of a haircut on Greece's debt is necessary to return the economy to growth, Germany, Greece's biggest European creditor, has staunchly opposed the idea.
A German finance ministry spokesman on Wednesday rejected measures that reduce the current value of Greek debt.
If Greece fails to convince its creditors by Sunday it will move a step closer to losing its euro membership, with a bond redemption by the ECB on July 20 possibly proving the trigger.
There is a 60 percent probability of Greece defaulting on the payment, according to economists in the poll.
"The moment that (ECB bond payment) does not get paid, it is a proper default. And that's the end because the ECB cannot continue justifying keeping Greek banks alive," Credit Suisse's Aranda-Hassel said.
The ECB has so far kept cash taps just about open for Greek banks through its emergency liquidity assistance program but, faced with a imminent default if a deal isn't sealed this week, it could cut off the flow. That would force Athens to print its own money and effectively leave the euro zone.
A Greek exit, the subject for heated debate for many months, will test the ECB's readiness to deal with possible contagion effects on sovereign bond markets in peripheral countries.
The ECB is buying mostly government debt worth 60 billion euros a month until at least September 2016, with the aim of keeping bond yields low and generating inflation.
But a majority of economists predict a Greek exit would force the ECB to alter its quantitative easing program, most likely by increasing the amount of its monthly purchases. (Polling by Sarmista Sen and Krishna Eluri; Additional reporting by Deepti Govind; Editing by Ross Finley and Catherine Evans)