A U.K. bookmaker has stopped taking bets on Greece leaving the euro zone, saying it is increasingly likely that the country could "begin the process of departing" very shortly.

William Hill closed their book on whether Greece will leave the euro zone during 2015, and on which country would be first to leave the euro zone, the bookmaker said Wednesday evening.

"Greece had been heavily backed down to 1/5 to be the first to quit the euro zone, and we'd also been shortening the odds for Greece to leave during 2015. They'd come down from 5/1 to 3/1.' William Hill spokesman Graham Sharpe said in a press release.

He added that "it is now looking increasingly likely that they could begin the process of departing very shortly."

It comes as one analyst told CNBC Thursday that the country faced a "slow-death scenario"—including a default and messy exit from the euro zone—as the country's economic crisis took another turn for the worse following a credit rating downgrade.

BofA Merrill Lynch Global Research's Thanos Vamvakidis warned Thursday that if Greece fails to reach a deal with its European partners, a Grexit—or Greek exit from the euro zone becomes inevitable.

His comments come after Greece's unresolved negotiations with its international creditors prompted ratings agency Standard & Poor's to cut its credit rating to "CCC+" from "B-" with a negative outlook.

"Without an agreement (with creditors over reforms), without official funding, there is a very high probability that Greece will default sometime in May and this could lead to a very negative scenario," Thanos Vamvakidis, head of European G10 FX strategy at BofA Merrill Lynch Global Research, told CNBC Thursday.

He said that although nobody wants that, "the more they delay the higher the risks."

"(A Grexit) is not going to be overnight. It would be a slow-death scenario and in a way we are in this scenario. Something needs to change in order to avoid an accident," he added.