MADRID — Spain’s borrowing costs neared record highs Monday as investors fretted over how the government would find additional money to bail out Bankia, the country’s largest mortgage lender, and other troubled banks.


Shares in Bankia plunged almost 30 percent early Monday after trading opened, only to recover some of the losses by mid-afternoon. The shares had been suspended Friday before a meeting of the bank’s board at which members called for an additional €19 billion, or about $24 billion, of capital after reviewing Bankia’s latest losses.


While the government in Madrid has insisted that it could afford to salvage Bankia and other troubled institutions without outside intervention, most analysts anticipate that ballooning mortgage defaults, coupled with Spain’s own punitive borrowing costs, will force Madrid to seek emergency funding for its banks from the European Union.


Spain seized control of Bankia on May 9, two days after replacing the bank’s management. Last Friday Bankia not only asked for billions in additional help but also revised its accounts, posting a 2011 loss of almost €3 billion instead of the €309 million profit that it reported in February.


Bankia’s collapse was described on Monday by Daragh Quinn, banking analyst at Nomura in London, as G.U.B.U. – grotesque, unbelievable, bizarre and unprecedented – using an acronym that was coined in Ireland, whose economy was sunk by the collapse of its banking sector.


Luis de Guindos, the Spanish economy minister, forecast this month that the total cost of rescuing Bankia and other Spanish banks would not exceed €15 billion. Mr Quinn from Nomura, however, estimated that recapitalizing Spanish banks would cost an additional €50 billion to €60 billion.


Madrid does not have that kind of money at hand, having depleted the funds that it had previously set aside to cover earlier meltdowns at some smaller banks. Bankia itself previously received a €4.5 billion emergency loan, bringing the total cost of its rescue to €23.5 billion, including the €19 billion requested Friday.


Meanwhile, the government could soon get saddled with three other banks — CatalunyaCaixa, Nova Caixa Galicia and Banco de Valencia — that have been put up for sale, so far unsuccessfully.


While the government could raise additional debt on the bond markets, it would now be paying near-record interest rates to do so. On Monday, the risk premium demanded by investors for holding 10-year Spanish government bonds rather than German bonds reached 510 basis points, the biggest differential since the introduction of the euro. A basis point is one-hundredth of a percentage point. The yield on Spanish 10-year bonds rose as high as 6.5 percent, close to the 7 percent level that triggered bailouts in Greece, Ireland and Portugal.


The government in Madrid, however, seems determined to avoid an international bailout of its banks — and in particular the conditions that would be set by creditors in return for such emergency funding. Instead, Madrid is looking at ways to tap further into financing from the European Central Bank, which has already helped sustain ailing banks in Spain and across Europe by providing them with three-year loans.


In earlier bailouts, governments made similar efforts to resist a bailout, albeit without the benefit at the time of the E.C.B.’s three-year financing program.


In the case of Portugal, for instance, Lisbon denied for months that it needed rescuing and instead pushed domestic banks to buy sovereign debt and compensate for the flight of foreign buyers. The strategy worked until April of last year, when the banks eventually said they would no longer purchase Portuguese government bonds, forcing Lisbon to seek a bailout within days.


Any failure to rescue Bankia could also turn what has so far been a trickle of domestic money leaving Spain into a full-fledged flight.


Meanwhile, Bankia has become a political hot potato for the government, both because of the failure to recognize earlier the extent of the banking problem and because of the losses suffered by investors who bought Bankia shares last July, when it was listed publicly.


The Socialist opposition party said over the weekend that it would oppose any further use of public funds to head off Bankia’s collapse until a full investigation took place to determine who had been responsible for the bank’s misstated accounts. The conservative government, however, has a parliamentary majority, and Alberto Ruiz Gallardón, the justice minister, suggested instead waiting for “the opportune moment” to review Bankia’s demise from a successful flotation to the biggest bailout in Spanish banking history.


“We have to handle these times thinking about what is best for Spain,” the minister said Sunday.