Spain's medium-term borrowing costs spiralled to a euro-era record at an auction on Thursday, hours before an independent audit was due to reveal how big a capital hole in Spanish banks needs to be filled by a euro zone bailout.

Euro zone finance ministers will discuss later in the day how to channel up to 100 billion euros ($126 billion) in rescue loans to Spanish lenders weighed down by bad loans from a burst property bubble. Many in the markets see the package as a mere prelude to a full programme for the Spanish state.

Spain's financial weakness is in focus a week before a European Union summit tackles long-term plans for closer fiscal and banking union in an effort to strengthen the euro's foundations, after bailouts for Greece, Ireland and Portugal failed to end a 2-1/2-year old debt crisis.

To pave the way for that, the leaders of Germany, Italy, France and Spain will meet in Rome on Friday.

Madrid sold 2.2 billion euros in medium-term bonds and attracted strong demand. But yields on 5-year paper rose to a 15-year high of 6.07 percent, a level regarded by analysts as unaffordable for any prolonged period.

The runaway Spanish yields contrasted with a French auction in which the yield on 5-year benchmark paper hit an all-time low of 1.43 percent.

"The first worry is can they (Spain) fund from the markets? They raised 2.2 billion versus a 2 billion target, so they can raise the money," said Achilleas Georgolopoulos, a strategist at Lloyds in London.

"Then the (question is), are the yields threatening for the medium term? And yes, clearly they are much higher than the previous auction ... But still they can continue for a few months to fund at these levels."

Two independent auditors are due to deliver a report to the Spanish government later on Thursday on the recapitalisation needs of the banking sector following last month's sudden nationalisation of Bankia, the fourth biggest lender.

Economy Minister Luis de Guindos will present the findings to euro zone colleagues meeting in Luxembourg from 1600 GMT. Banking sources believe the report will say the lenders need to raise a further 60-70 billion euros.

However, a formal request for assistance may only come on Friday when Prime Minister Mariano Rajoy meets German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Mario Monti.

The finance ministers will discuss which of the euro zone's rescue funds - the temporary European Financial Stability Fund or the permanent European Stability Mechanism - will lend Spain the money. This matters to investors because ESM loans would be senior to other Spanish borrowing, meaning private bondholders would face first losses in any debt writedown.

The ministers are also expected to ponder the next steps with Greece, following the formation of a coalition of mainstream parties committed to the country's 130 billion euro EU/IMF bailout but determined to renegotiate some of the terms.

Athens will ask lenders for two more years to hit fiscal targets and an extension to unemployment benefits as it seeks to soften the punishing terms of the bailout saving the country from bankruptcy, a party official said.

Greek officials have said this would entail an extra 16-20 billion euros in foreign funding. It sets up a showdown with Greece's euro zone partners, in particular paymaster Germany, which have offered modifications but no radical re-write of the conditions attached to the lifeline agreed in March.

"We can always discuss conditions of the loan. But let us not forget one thing: This is not one-way development aid," Luxembourg Finance Minister Luc Frieden told Reuters Insider television.

The German government and opposition reached a deal on growth that will allow parliament to approve the euro zone's permanent bailout scheme next week, but Germany's top court may delay the rescue fund's start date scheduled for July 1, saying it needed time to study the treaty.

The ESM cannot go into effect without approval by Europe's biggest economy. Ratification also requires the signature of the president and a nod from the constitutional court in Karlsruhe.

The euro zone ministers may also consider a suggestion by Monti, made on the sidelines of this week's G20 summit, to use the euro zone's rescue funds to buy the bonds of Spain and Italy in the secondary market to bring down their borrowing costs.

Merkel has played down the idea, which investors said might be counter-productive unless the ECB stepped in decisively in support.

Any cash injection would come with strings attached, equivalent to the sort of bailout programmes that Italy and Spain are trying to avoid because of the stigma attached.

"Right now we have no requests to buy via the EFSF/ESM, Spanish or Italian bonds," Frieden said. But he added: "We have funds available if necessary to help those countries in need ... We will do whatever is necessary to make sure that this zone, which is in trouble, will remain in the medium and long term a stable monetary union." - Reuters