ATHENS/BRUSSELS (Reuters) - Greece's budget gap last year was worse than feared, the European Union's statistics office revealed yesterday, as Moody's Investors Service downgraded its rating of Greek government debt.

The news triggered a fresh slide of asset prices in Greece and other debt-choked European countries, and increased pressure on Athens to seek billions of euros of emergency loans from the EU and the International Monetary Fund.

Greece's two-year government bond yield soared four percentage points to 12.26 percent as investors bet the country would need a bailout to avoid restructuring its debt or defaulting. Athens will have to refinance 8.5 billion euros ($11.3 billion) of bonds maturing on May 19.

Yesterday evening, Prime Minister George Papandreou was in the seventh straight hour of talks with ministers on how to handle the crisis. It was unclear whether an announcement would be made after the meeting.

"It looks like a terrible situation just got worse," said Nick Kounis, economist at Fortis.

The budget figures were announced as tens of thousands of Greek nurses, teachers and other public workers staged a one-day strike to protest against the government's austerity measures.

More than 10,000 civil servants and students marched to parliament, demanding that Athens reject any pressure for further spending cuts in crisis talks that it launched this week with the EU and the IMF.

The Greek government posted a budget deficit of 32.34 billion euros or 13.6 percent of gross domestic product in 2009, not the 12.7 percent which it had reported earlier, Eurostat said in a review of countries' deficits throughout the region.

It added that the Greek deficit might be revised again, by between 0.3 and 0.5 percentage points of GDP, because of uncertainty about the quality of Greece's data and accounting procedures.

In a brief statement, the Greek Finance Ministry insisted the new numbers would not change its intention to shrink the deficit by four percentage points this year. It said measures already taken would be enough to cut the deficit by six points.

But both Athens and EU officials appeared to be backing away from a previously announced target for Greece to slash the deficit to 8.7 percent of GDP this year.

"The target for 2010 is a four percentage point reduction of the deficit. We did not refer to the starting point or the arrival figure, only the reduction effort," European Commission spokesman Amadeu Altafaj said in Brussels. "Greece is on track to meet the target for 2010; that is what counts."

Some revision to the 2009 budget gap had been expected, and several analysts said Athens might still succeed in cutting its deficit sharply this year.

But the financial markets were hit hard by the revision because inaccuracies in Greek data — some of them apparently deliberate and politically motivated — have fuelled its debt crisis by angering investors and Greece's EU partners.

Last October, the incoming socialist government said Greece's 2009 budget deficit would be twice as big as a previous estimates — and four times the EU ceiling.

"What concerns me is the general uncertainty about the Greek official figures. This affects market perception about Greece ...that one can't rely on the Greek statistics and that the deficit is revised up and up and up," Giada Giani, economist at Citigroup, said yesterday.

The Greek Finance Ministry attributed the latest revision to a deep recession, which reduced GDP more than expected, and a reassessment of the financial accounts of pension funds.

Moody's cut Greece's sovereign rating by one notch to A3, placing it four notches above "junk" status, and kept the new rating on review for a possible further downgrade.

"There is a significant risk that debt may only stabilise at a higher and more costly level than previously estimated," it said. Rival agencies rate Greece lower, with Standard & Poor's at BBB+ and Fitch at BBB-.

Greece's two-year government bond yield has soared nearly 11 percentage points from just 1.38 percent before the crisis erupted last November. The 10-year bond yield hit 9.17 percent yesterday but rose more slowly, increasing the inversion of the Greek yield curve — a classic sign that investors fear Greece may have trouble servicing its debt.

The cost of insuring five-year Greek government debt against a default through credit default swaps shot up to the highest level in Europe, surpassing Ukraine.

Investors fear other weak euro zone states could become the next "dominos" if Greece defaults; bond yields and CDS for Spain, Portugal and Ireland also rose yesterday.

The euro sank almost one percent to near one-year lows against the dollar, as investors worried that the Greek crisis had exposed severe economic strains within the euro zone and member states' difficulties in coordinating policies.

The markets think Greece will almost certainly have to apply for a 40-45 billion euro aid package from euro zone states and the IMF — though the German public's opposition to helping Greece could delay the disbursal of funds by Berlin.