The British Prime Minister David Cameron has called on eurozone countries to take decisive action to stem the debt crisis and said the people of Greece must decide if they want to stay in the euro.

Speaking in Washington ahead of the G8 summit Mr Cameron said that Britain would not accept a Europe-wide financial transaction tax, which is supported by the French President Francois Hollande.

Mr Hollande held talks with the US President Barack Obama at the White House on Greece and the eurozone debt crisis and said that Greece must stay in the euro.

Elsewhere, the European Commission and the European Central Bank are working on scenarios in case Greece has to leave the eurozone.

Concern has grown that Greece may decide to leave or be forced out of the eurozone after a rise in popular opposition to an EU-IMF programme of fiscal austerity and structural reforms.

EU trade commissioner Karel De Gucht in a newspaper interview said that "today there is in the European Central Bank, as well as in the Commission, services working on emergency scenarios if Greece shouldn't make it."

Speculation about such planning has been rife, but the comments appear to be the first time an EU official has acknowledged the existence of contingency plans being drawn up in case Greece has to drop out of the currency bloc.

A source said Mr De Gucht was explaining that EU institutions had not been sitting on their hands for the past two years, and that they were now better prepared than they had been.

Greeks are scheduled to go to the polls again on 17 June after a failure to form a government following elections on 6 May.

A victory by the far-left, anti-bailout coalition SYRIZA - which some opinion polls suggest is likely - would increase the possibility of the country leaving the euro.

However, one opinion poll yesterday showed the pro-bailout New Democracy party in first place, several points ahead of the SYRIZA, which has pledged to tear up the bailout agreement.

The euro fell to a four-month-low and European stock markets fell in early trade this morning, following a ratings downgrade for Greece and for 16 Spanish banks.