European officials are working on contingency plans in case Greece bombs out of the euro zone, the EU's trade commissioner said, while Berlin said it was prepared for all eventualities.

German Finance Minister Wolfgang Schaeuble, one of Greece's harsher critics, said market turmoil fuelled by the euro zone debt crisis could last another year or two.

'Regarding the crisis of confidence in the euro ... in 12 to 24 months we will see a calming of the financial markets,' he said.

European shares hit their lowest level since December, depressed by the prospect of a Greek euro exit spreading a wave of contagion in the currency bloc which could engulf much larger economies such as Spain's.

Policymakers insist they want Greece to remain in the euro zone but European Union trade commissioner Karel De Gucht said the European Commission and the European Central Bank were working on scenarios in case it has to leave.

'A year and a half ago there maybe was a risk of a domino effect,' De Gucht told Belgium's Dutch-language newspaper De Standaard.

'But today there are in the European Central Bank, as well as in the Commission, services working on emergency scenarios if Greece shouldn't make it. A Greek exit does not mean the end of the euro, as some claim.'

Speculation about such planning has been rife, but De Gucht's comments appeared to be the first time an EU official has acknowledged the existence of contingencies being drawn up. A German finance ministry spokeswoman, asked about plans for a possible Greek exit, said without elaborating: 'The German government naturally has the responsibility to its citizens to be prepared for any eventuality.'

But a spokesman for the European Commission, the EU's executive, wrote on Twitter there was no active planning.

'(The) European Commission denies firmly (that it) is working on an exit scenario for Greece,' Oliver Bailly wrote. '(The) commission wants Greece to remain in the euro area.'

World shares slid and German borrowing costs hit record lows as uncertainty about Greece's future in the euro zone and a deepening Spanish banking crisis bolstered safe-haven assets.

Investors were rattled by a ratings downgrade of 16 Spanish banks by Moody's Investors Service, although the move had been expected. Sentiment has soured to such an extent that an opinion poll showing Greeks are returning to establishment parties which support the country's bailout had little impact. If they vote that way in June 17 elections, Greece's place in the euro zone would look more secure and the threat of contagion would diminish.

The poll, the first conducted since talks to form a government collapsed and a new election was called, showed the conservative New Democracy party in first place, several points ahead of the radical leftist SYRIZA which has pledged to tear up its 130 billion euros bailout programme.

'It's up to Greek politicians to explain the reality to their people and not make false promises,' Schaeuble told France's Europe 1 radio. 'We want Greece to stay in the euro but meet its commitments and that's a decision that's up to the Greeks.'

The fact the euro zone crisis is moving back into an acute phase will place it centre stage at a weekend summit of leaders of the G8 top industrialised nations.

President Barack Obama, the G8 host, has urged European leaders repeatedly to do more to stimulate growth, fearing contagion from the crisis could hurt the US economy and his chances of re-election in November.

New French President Francois Hollande is pressing for measures to boost growth rather than cut debt, Britain's David Cameron has become increasingly vocal in demanding Europe act more decisively, Canada's Stephen Harper has been a frequent critic, and of the euro zone G8 members, Italian premier Mario Monti was calling for pro-growth policies before Hollande was.

That could leave Germany's Angela Merkel, who insists debt-cutting programmes cannot be diluted, cutting a lonely figure.