D.C. Chief Financial Officer Jeffrey S. DeWitt, left, says the budget adopted by D.C. lawmakers could jeopardize the city’s credit rating. (Jahi Chikwendiu/The Washington Post)

D.C. Chief Financial Officer Jeffrey S. DeWitt said Monday he will not certify the budget approved by D.C. lawmakers last month, asserting that the spending plan improperly diverts money that should help repay the city’s debts for the Walter E. Washington Convention Center.

DeWitt’s decision, conveyed in a letter to D.C. Council members, comes after a prolonged standoff between the chief financial officer and council Chairman Phil Mendelson (D). DeWitt had repeatedly warned that he would reject the $15.5 billion budget, but the council voted to move ahead over his objections.

The decision throws the city’s budget process into chaos at a time when spending for the coming fiscal year is ordinarily finalized. Its immediate effect could be the loss of tens of millions of dollars for urgent repairs to the District’s dilapidated public housing complexes — money that Mendelson sought to divert from the Washington Convention and Sports Authority’s reserves.

It was not immediately clear how the council will respond to DeWitt’s letter. In a brief statement Monday evening, Mendelson said he did not agree with the decision. His spokeswoman said he was in meetings with council budget officials.

D.C. Mayor Muriel E. Bowser (D) did not immediately respond to requests for comment.

Under D.C. law — which grants expansive powers to the chief financial officer, a legacy of the era when politicians’ chronic fiscal mismanagement led to a federal takeover of the city’s budget — DeWitt must certify the budget before it can be submitted to the mayor or Congress for approval. That could require the council to vote on new legislation that would satisfy his concerns.

D.C. Council member Jack Evans (D-Ward 2), first elected in 1991 and chairman of the Finance and Revenue Committee, said he could not remember a similar showdown between the council and chief financial officer during his time in office.

“I understand the chairman’s interest in identifying funding for some very important programs in the District, and his rationale for doing so,” Evans said. “But it is clear from the CFO’s letter . . . that this would not pass muster.”

He added, “We’re in uncharted territory for this.”

The budget passed by the council would remove $49 million from the convention center’s reserves. Half of that money would go to public housing repairs, while the other half would eliminate the need for an increase in hotel room taxes proposed by Bowser.

DeWitt argues that the transfer would violate the city’s commitments to bondholders, whose investments funded the construction of the convention center in 1998 and its hotel in 2010, and who expect excess revenue to go toward repayment.

The chief financial officer acknowledged in his letter that his office identified excess dollars in the convention center authority’s budget but said only about $20 million of that money is immediately available for other purposes.