(Adds data on deficit, background on debts to state-owned banks and pension funds)
BRASILIA, Dec 28 (Reuters) - Brazil's central government posted a record monthly shortfall in a key budgetary gauge watched by creditors, raising questions over President Dilma Rousseff's ability to deliver on her pledge for a 2016 surplus amid a recession.
Excluding debt service, expenses at ministries, the central bank and social security topped revenue by 21.279 billion reais ($5.47 billion) last month, up from a 12.27 billion reais deficit in October, the National Treasury said on Monday.
The deficit was the largest since the beginning of the data series in 1997.
So far this year, the central government amassed a primary deficit, or the difference between revenue and expenses before interest payments on the debt, of 54.3 billion reais, the biggest year-to-date figure ever.
The primary budget balance is scrutinized by investors and credit rating agencies as a gauge of creditworthiness.
Earlier this year, Standard & Poor's and Fitch stripped Brazil of its investment-grade rating and threatened further downgrades next year if the government fails to produce primary surpluses to curb its debt burden.
The primary deficit could widen considerably as the government prepares to repay this week around 57 billion reais in debts with state-controlled banks and state workers' pension funds.
Interim Treasury Secretary Otavio Ladeira said the government will use mostly resources from an account it holds at the central bank to repay the debt. The government will only issue 1.5 billion reais in notes to repay Banco do Brasil SA , the nation's largest lender by assets.
Finance Minister Nelson Barbosa is seeking to repay those debts this year to avoid hampering fiscal results in 2016.
Barbosa, named to the post in Dec. 18, is scheduled to meet Rousseff later on Monday to discuss the repayment.
For next year, Rousseff has vowed to produce a central government primary surplus of 24 billion reais.
However, many analysts doubt Rousseff will be able to meet that goal next year as the recession extends into 2016 and a political stalemate blocks the passage of legislation to fix public finances.
The central bank is scheduled to publish data on the consolidated primary budget balance, which includes the results of regional governments and state companies, on Tuesday.
$1 = 3.8917 Brazilian reais Reporting by Marcela Ayres; Writing by Alonso Soto; Editing by Chizu Nomiyama, Andrew Hay and W Simon