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MADRID—Spain has been quietly tapping the country's richest piggy bank, the Social Security Reserve Fund, as a buyer of last resort for Spanish government bonds, raising questions about the fund's role as guarantor of future pension payouts. Now the scarcely noticed borrowing spree, carried out amid a prolonged economic crisis, is about to end, because there is little left to take. At least 90% of the €65 billion ($85.7 billion) fund has been invested in increasingly risky Spanish debt, according to official figures, and the government has begun withdrawing cash for emergency payments. Although the trend has drawn little public attention or controversy, it has become a matter of concern for the relatively few independent financial analysts who study the fund, which is used to guarantee future payments of pensions. They say the government will soon have one less recourse to finance itself as it faces another year of recession and painful austerity measures to close a big budget deficit. That pressure, some analysts said, could force Prime Minister Mariano Rajoy's government to seek a rescue this year from the European Union's bailout fund, a politically risky course he seeks to avoid. In addition, there are worries that Social Security reserves for paying future pensioners are running out much quicker than expected. In November, the government withdrew €4 billion from the reserve fund to pay pensions, the second time in history it had withdrawn cash. The first time was in September, when it took €3 billion to cover unspecified treasury needs. Together, the emergency withdrawals surpassed the legal annual limit, so the government temporarily raised the cap. Demonstrators including a retired couple and their daughter marched at a protest against tax increases and austerity cutbacks in Malaga, southern Spain, in October. Spanish officials defend the heavy investment of the Social Security Reserve Fund in their government's high-risk bonds. They say the practice is sustainable as long as Spain can continue borrowing in financial markets, and they predict the economy will start to recover late in 2013, easing the debt crisis. But some analysts say Spain will have trouble finding buyers for the estimated €207 billion in debt it plans to issue in 2013, up from €186 billion in 2012, to cover central-government operations, debt maturities of 17 regional administrations, and overdue energy bills. "With foreign investors staying away from the Spanish debt market, you're going to need all the support you can get from domestic players," said Rubén Segura-Cayuela, an economist with Bank of America-Merrill Lynch. And domestic appetite for Spanish debt, he added, may not be enough. Spain's commercial banks already have increased their Spanish government-bond portfolio by a factor of six since the start of the crisis in 2008, and now own one-third of government bonds in circulation. The percentage of Spanish government debt held by the Social Security Reserve Fund stood at 55% in 2008, according to official figures; by the end of 2011 it had risen to 90%. Analysts say the percentage has continued to rise, even as international agencies have lowered Spain's credit ratings.