(Adds overall deficit, analyst comment and data)
By Alonso Soto
BRASILIA, Jan 29 (Reuters) - Brazil's overall budget deficit soared to a record 613 billion reais ($150.99 billion) in 2015, central bank data showed on Friday, nearly doubling from last year as efforts to rebalance fiscal accounts failed and interest rates shot up.
The budget deficit equaled 10.34 percent of the gross domestic product, nearly five times its shortfall in the 12 months to mid-2011. The deficit mushroomed under President Dilma Rousseff, who took office at the start of 2011.
In comparison, at the height of its debt crisis in 2009 Greece had a deficit of 15.2 percent of GDP.
Brazil's widening budget deficit is fueling worries among investors that the once-booming economy may struggle to repay its debt in coming years as a deepening recession cuts revenues. On the other hand, high inflation has forced authorities to raise interest rates, the rate that the government pays some investors who purchase its debt.
Measures to cut spending and raise revenues failed to provide enough savings to close the widening budget gap under pressure from rising law-mandated expenditures linked to health and education
The closely monitored primary budget balance, or revenues after expenditures prior to debt interest payments, ballooned to a record 111.249 billion reais ($27.4 billion) in 2015.
The country posted a primary deficit of 71.729 billion reais ($17.6 billion) in December, above market expectations for a gap of 65.15 billion reais.
The rapid deterioration of Brazil's finances has significantly raised the country's debt burden and caused it to lose its coveted investment-grade rating last year.
Rousseff has vowed to rebalance the public accounts and return to a modest primary surplus this year, but most analysts doubt she will be able to fulfill her promise as revenues continue to plunge while spending remains constant.
"Meeting this year's target looks very challenging," said Alberto Ramos, senior economist with Goldman Sachs. "The fiscal problem goes beyond the target, what is needed are changes to the medium and long term fiscal trajectory... more structural fiscal reforms."
Brazil's gross debt shot up to 66.2 percent of GDP, up sharply from 57.2 percent in 2014.
In the 12 months through December, the primary budget deficit widened exploded to the equivalent to 1.88. percent of GDP from 0.89 percent in the year ended in November.
$1 = 4.0600 Brazilian reais Additional reporting by Marcela Ayres; Writing by Silvio Cascione; Editing by W Simon