Bookmaker William Hill offered odds of 4/6 Thursday that Greece will cease to use the euro by the end of 2012, the first time it has bet odds-on that Athens will exit the currency.

It came after Greek Prime Minister George Papandreou's shock decision earlier this week to offer a referendum on the euro, which has stunned fellow eurozone nations and sparked a revolt among his allies in Athens.

"For the first time we are betting odds-on that the Greeks will quit the euro," said William Hill spokesman Graham Sharpe in a statement.

They offered odds of 11/10 that the Greeks will still be using the euro on December 31, 2012.

Leaders of the world's most powerful economies also began a two-day G20 summit in the French Riviera resort of Cannes on Thursday dominated by the debt crisis.

Papandreou was summoned to pre-summit talks Wednesday with French President Nicolas Sarkozy and German Chancellor Angela Merkel, where he agreed to hasten the referendum.