By Josie Cox 

Greek government bonds plunged Thursday, shaken by swelling fears that the beleaguered country will be forced into a default.

Yields on the country's two-year bonds soared by more than 4 1/2 percentage points on the day to above 27%--their highest since being issued and a massive jump even for notoriously volatile Greek debt. Yields rise when prices fall.

On the country's 10-year debt, meanwhile, yields advanced more than one percentage point to a shade over 13%--their highest in more than two years.

An inverted yield curve, where shorter-term debt yields more than longer-dated bonds, is a classic signal that investors see a very high risk of default.

"Overall the probability of a Greek exit [from the euro] remains higher now than it ever was," economists at Barclays wrote in a note. Others warned that a default or an exit from the euro could spark a wave of volatility across other Southern European countries, such as Spain, Italy and Portugal, that were hit hardest during the height of the eurozone crisis.

Late Wednesday, Standard & Poor's Ratings Services slashed its credit ratings on Greece to triple-C-plus, saying it expects the country's debt and financial commitments to be unsustainable without deep economic reform.

"The outlook for full-year economic growth is highly uncertain," S&P wrote, adding that it doesn't expect the stalemate between Greece and its official lenders to be resolved before the middle of May.

Greece is due to repay the International Monetary Fund more than EUR750 million ($801 million) on May 12--part of the terms of its first bailout program in 2010.

"If I were an investor, I think I'd even be tempted to ditch some Portuguese and Spanish bonds in this risk environment," said Neil Mellor, a strategist at Bank of New York Mellon. "It could be a case of the market not understanding how bad the situation is until it is too late," he added.

J.P. Morgan economist Malcom Barr said that his base case remains that Greece and its creditors will come to an agreement that prevents a default but a deal would only be struck "at the 11th hour, when the country is staring a default in the face."

Thursday's situation marks a sharp turnaround from a year ago, when Greece sold more than $4 billion worth of bonds for which investors put in orders worth around $28 billion, according to bankers.

Strategists at the time said that the transaction--the first of its kind for the country since April 2010--had paved the way for Greece's return to funding markets faster than expected.

On Thursday, though, the bonds were yielding just over 19% according to Tradeweb, having priced almost exactly 12 months ago at 4.95%.

Among the buyers of the bond at the time, which matures in 2019, were Atlanta-based asset-management firm Invesco, as well as Legal & General Investment Management and BlackRock Inc.

Legal & General declined to comment on whether it still hold those bonds and BlackRock wasn't available for comment.

Invesco sold "most" of the bonds that it had bought in April last year in September, on account of "political risks," said Nicholas Wall, a London-based portfolio manager at Invesco, which has around $798.3 billion under management.

He said that Invesco now only has very marginal exposure to Greece, though declined to give a specific number, and would consider becoming more bullish--or optimistic--on the country only if there is a change in composition of the government.

"Of course there is a price for everything, but in this case it's very hard to determine where that might be for Greek bonds and what risk premium I would need to see before thinking about buying," said David Vickers, a multiasset portfolio manager at Russell Investments, which has around $272.8 billion in assets under management.

He added that the situation lacks clarity and precedent and that despite the fall in prices, he isn't yet considering buying. Russell Investments currently has no direct exposure to any Greek assets.

Earlier this week, Valdis Dombrovskis, vice president of the European Commission in charge of euro and social dialogue, bemoaned the lack of progress in talks. "It is important for the Greek side to step up its efforts," he said.

Greece's Syriza-led government has been locked in negotiations with its international creditors since coming to power in late January. It needs a deal by this summer to secure billions of euros in bailout aid to avoid defaulting on its debts and potentially exiting the euro.

Greece's main stock index flitted between slim gains and small losses on Thursday, following a sharp selloff on Wednesday.

So far this year, the Athex index has declined more than 10%. Over the past 12 months losses have surpassed 39%, making it one of the world's worst-performing equity indexes.

Data late last month showed that Greek bank deposits plunged to an almost 10-year low in February, as investors yanked out some EUR8 billion.

"Greek banks [could soon] require more serious policies, like capital controls," said Paul Donovan, an economist at UBS.

Jitters over Greece have also helped to fuel a hunt for safe assets in Europe helping to propel German government bonds to record highs.

Write to Josie Cox at josie.cox@wsj.com

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