Under a measure approved by voters last year and signed by Mayor Vincent C. Gray, the District no longer needs to submit its budget to the president and Congress for approval. (Marvin Joseph/The Washington Post)

The D.C. Council will sue Mayor Vincent C. Gray and the city’s chief financial officer, the council chairman said Wednesday, setting up the first such legal showdown between the city’s two branches of government in a decade.

Council Chairman Phil Mendelson (D) said the council will ask a D.C. Superior Court judge to determine whether Gray (D) and CFO Jeffrey S. DeWitt are violating a voter-approved law that allows the city to spend billions of dollars of its own money without strict congressional approval.

Under the measure approved last year — which was signed by Gray and passed a congressional review period — the District no longer needs to submit its budget to the president and Congress for approval. The process left the city vulnerable to national politics and often complicated its financial planning.

Now, the budget would pass the council, just as any other city legislation, and it would take effect unless Congress voted to reject it and the president agreed.

But last week, DeWitt joined Gray, Attorney General Irvin B. Nathan and the Government Accountability Office in saying the measure had no legal effect because it violates the city’s charter, set by Congress.

Mendelson and a team of pro bono lawyers disagree. In a suit they intend to file Thursday, the council argues that Gray and others have been relying on a flawed legal analysis in rejecting the measure.

The section of law that the measure amended, they say, was one Congress did not set in stone but left subject to ­changes.

“We believe that the law is valid, and we believe it is inappropriate to essentially invalidate it,” Mendelson said of the measure, which he wrote. “This disagreement should be resolved in the courts.”

Gray and Mendelson have been at odds over the issue since last month when the chairman set a series of budget hearings and votes extending into June.

That would miss a traditional deadline for Gray to submit the budget to the president.

Last week, Gray and DeWitt admonished Mendelson to continue as planned.

Nathan, in an April 8 memorandum, called the measure “a nullity, with no legal force or effect,” echoing language used by the GAO in its January opinion. The only way for the District to change its financial relationship with Congress, both said, is through an act of Congress.

But according to a summary of the lawsuit provided to The Washington Post, the council will argue that those interpretations are flawed and that the measure properly amended the charter, allowing the city to spend locally raised funds without a congressional appropriation.

Mendelson said he believed each party is hardened in its position on the law’s validity and that the courts are the only way to resolve the dispute.

“The sooner this is filed, the sooner this is resolved,” he said.

Karen L. Dunn, a partner at Boies, Schiller & Flexner LLP, and Brian Netter, a partner at Mayer Brown, are representing the council free of charge. Dunn said the council will ask for an expedited review and a final decision — including appeal — by May 28, the day the council is scheduled to take its first vote on the $10.7 billion plan.

It is not the first time the council has sued the mayor. In 1992, Council Chairman John A. Wilson filed suit against Mayor Sharon Pratt after she defied a council law on the review of contracts, arguing that it exceeded the council’s authority under the charter. Pratt ultimately prevailed.

And in 2003, Council Chairman Linda W. Cropp sued Mayor Anthony A. Williams over another separation-of-powers dispute, involving the appointment of the District’s inspector general. That case was rendered moot and dismissed.

Mendelson said he did not consider his lawsuit adversarial in the same way, noting that Gray, DeWitt and Nathan have all said they support the ultimate goal of obtaining budget autonomy.

“This is not the government fighting with itself,” he said. “This is the government struggling to find the right answer. . . . The only way we find out is by going to court.”