Spain's borrowing costs shattered euro-era records Thursday after Moody's downgraded its debt close to junk-bond status and warned of a growing risk of a full-blown bailout.

Pie-chart showing the amount of Spanish public and private debt held by foreign banks.

Instead, the sheer size of the rescue loan fed fears about its impact on Spain's mushrooming sovereign debt and prompted concerns that the state may eventually need rescuing itself.

The Spanish crisis is unfolding just three days before Greek elections, further destabilising markets that fear a victory by anti-austerity parties could send Athens back to the drachma.

The interest rate on Spanish 10-year government bonds soared to 6.9650 percent, the highest since the birth of the single currency, from 6.721 percent the previous evening.

Such high interest rates are regarded by many analysts as impossible for the nation to afford to finance its activities over the longer term, raising the risk of a bigger bailout, just as was the case for Greece, Ireland and neighbour Portugal.

Moody Investors Service slashed Spain's sovereign debt rating by three notches late Wednesday to Baa3 and left it on review for a possible further downgrade.

Since many institutional investors are barred from buying bonds that are rated as junk, or non-investment grade, the prospect of a further downgrade sent a further chill through the market.

The difference in the rate between Spanish and safe-haven German 10-year bonds widened to a high of 5.43 percentage points, flirting with last week's euro-era record of 5.48 percentage points.

"The risk of losing investment grade pressured the differential this morning and left it at historic highs," analysts at Spanish brokerage Renta 4 said in a market report.

Spain's government insists the eurozone rescue loan, destined to recapitalise bank whose books are heavily exposed to a 2008 property market crash, will be repaid by those lenders who receive the money.

But the final responsibility for repaying the eurozone is clearly Spain's, Moody's said.

"This will further increase the country's debt burden, which has risen dramatically since the onset of the financial crisis," the agency said in a statement.

It predicted Spain's public debt, which amounted to 68.5 percent of economic output at the end of 2011, would bulge to 90 percent this year and carry on rising until the middle of the decade.

At such high borrowing rates Madrid had limited access to the markets, as demonstrated by the fact that it was forced to seek a rescue loan, the agency said.

Further, the country was increasingly dependent on commercial banks, flush with cheap loans provided the European Central Bank, to buy its government bonds, the agency said.

"In Moody's view this is an unsustainable situation."

Unless there were signs of a quick improvement in Spain, such as an exit from recession or rapid progress in cutting the deficit, "neither of which is likely," the agency warned that Spain would be increasingly constrained in refinancing maturing debt.

If unchecked, Moody's said Spain could lose affordable access to the debt markets and face a rising risk of a full-blown state bailout by the European Financial Stability Facility and/or the new European Stability Mechanism, which comes into force next month.

"Moody's action to place the government's rating one notch above speculative grade reflects the rating agency's view that Spain has moved much closer to needing to seek direct support from the EFSF/ESM, and therefore much closer to being positioned within speculative grade," it said.

If the Spanish state requires a rescue, investors could be burned, the agency said.

"Moody's believes that the debts of euro area sovereigns that are fully dependent upon official sources to fund their borrowing requirements represent speculative-grade risk," it said.

"Support would, if needed for a sustained period, be likely to be made conditional on loss-sharing with private investors or in extremis withdrawn altogether."