MADRID  - Spain, which is under market pressure to request a financial bailout for its economy, faces a key test of investor confidence on Thursday when it auctions long-term government bonds.Madrid hopes to take advantage of a fall in its borrowing costs since the European Central Bank detailed plans earlier this month to buy the bonds of struggling eurozone countries like Spain that submit to strict economic conditions. But ECB intervention is dependent on Spain asking for help via the region's bailout fund -- a step it appears reluctant to take, particularly if bond yields remain at affordable levels."We are studying the situation, we are going to see if this is good for us or not, if it is necessary or not," Prime Minister Mariano Rajoy said last week, adding the decision would depend in part on bond yield levels.Spain's Treasury will try to auction a total of between 7.0 billion euros and $9.2-11.8 billion of T-bills and bonds this week. On Tuesday it raised 4.6 billion euros, slightly more than planned, in short-term debt at rates down from August. But the real test comes on Thursday when the Treasury will try to raise a similar amount in an auction of 3-year and 10-year bonds."Bond auctions have gone well in recent weeks. We would expect a reasonable reception right now" by investors to Thursday's bond auction, said Marian Fernandez, a strategist at investment group Inversis.But many analysts warn Spain's borrowing costs could skyrocket to unsustainable levels if the country continues to put off asking for aid from the ECB, leaving Madrid with no choice but to ask for a financial bailout.Jean-Francois Robin, global head of strategy at Natixis, said "the temptation could be to do nothing" because of the recent drop in yields."Spanish yields have dropped in anticipation of an intervention by the ECB and Madrid does not have much inclination to formally demand aid as long as they remain low," he said in a research note."The risk is that the exit from this situation will involve a rise in yields that will force Madrid's hand," he added.Spain's 10-year bond yields on Monday climbed above 6.0 percent, a level at which many economists believe the long-term sustainablity of a country's debt becomes a concern, after having slid following the ECB announcement.Daily El Pais reported Monday that Economy Minister Luis de Guindos wants Spain to request aid right away while Rajoy wants to do everything to avoid asking for help, which he fears would have a huge political price.Madrid has already accepted a eurozone rescue loan of up to 100 billion euros to save its banks, still reeling from a 2008 property market crash."It's obvious that investors all have one expectation which is that Spain will get a bailout. Reversing this situation would be very costly in terms of credibility," said Alberto Roldan, director of market analysis at Spanish brokerage Inverseguros.Spain faces a series of crucial dates regarding the economy over the coming weeks.The government will unveil on September 27 its budget for next year, which is expected to include more austerity measures to rein in the public deficit.The following day it will present the results of the final audit of the country's banking sector.Spain also faces debt repayments of about 30 billion euros in October."There is still a problem of liquidity and we are approaching October's debt repayments. The next step is up to Spain. If we don't ask for any financial aid or bailout, the ECB will not intervene to buy debt," said Fernandez.