A key congressional agency has declared that an attempt by the District to win more autonomy from Congress through a voter referendum is not legal, setting back the latest strategy to increase the city’s independence from Capitol Hill.

More than 80 percent of D.C. residents voted in an April referendum to back the “budget autonomy” measure, which was intended to allow the city to spend locally raised tax revenue without a congressional appropriation. It has been promoted by voting-rights activists as a way to give the city new rights without having to navigate congressional gridlock.

But the Government Accountability Office, Congress’s investigative arm, said Thursday that in holding the referendum, city officials exceeded their powers under the home rule charter approved by Congress in 1973.

“While the Home Rule Act grants the District Government substantial powers of self-government, the District Government must also comport with all of the act’s limitations,” wrote Susan A. Poling, the agency’s general counsel. She said portions of the ballot proposal “that purport to change the federal government’s role in the District’s budget process are without legal force or effect.”

Those who backed the measure played down the opinion, saying it had no bearing on the local officials who prepare and execute the city’s budget. “Nothing changes from our perspective,” said Kimberly Perry, executive director of the nonprofit group D.C. Vote, which played a leading role in developing the referendum strategy. “An opinion is just an opinion, and it does not change the law.”

Perry said that only a court ruling or an act of Congress signed by the president could reverse the measure. “Otherwise we fully expect District officials to move forward implementing this law,” she said.

That view was endorsed by the District’s delegate to Congress. “While GAO opinions are certainly well respected in Congress, they have no legal effect,” said Del. Eleanor Holmes Norton (D-D.C.), who also pledged to fight any attempts to overturn the referendum through federal legislation.

The agency’s review was done at the request of the House Appropriations subcommittee that oversees the District’s budget. House Republicans have been skeptical of the referendum, calling it an “expression of the opinion of the residents, only, and without any authority to change or alter the existing relationship between Federal appropriations and the District” in a July report.

Thursday’s opinion is likely to harden that view, and it could guide Congress’s reaction if local leaders proceed with implementing the law. Jennifer Hing, a spokeswoman for the House Appropriations Committee, said the panel “concurs with the GAO’s findings.”

Although the opinion may not be legally binding on District officials, it is likely to influence the way Chief Financial Officer Jeffrey S. DeWitt approaches the issue in the coming months. David Umansky, a spokesman for DeWitt, declined to comment Thursday.

Among other effects, a budget autonomy law would allow the city government to set a different fiscal year from the federal government’s and to continue normal operations during a federal shutdown.

The effect of last fall’s federal shutdown on the city was the subject of a Senate hearing Thursday. Sen. Mark Begich (D-Alaska) said it was clear that any future stalemates should not risk shutting down municipal services in the nation’s capital.“What inaction or action we take here in Congress should not impact a city at the magnitude we do your city,” said Begich, chairman of a Homeland Security and Government Affairs subcommittee that oversees D.C. matters. “It shouldn’t cause a shutdown of a city, especially right where it is critical to keep the functions moving.”

Because Congress granted the District the ability to spend local funds through September 2015 in a recent spending bill, the referendum will have no practical effect before then.

Virtually all District leaders and congressional Democrats, as well as some influential Republicans, have expressed support for pushing budget autonomy legislation through Congress. But Republican attempts to attach measures dealing with city laws on abortion, gun control and other issues have kept a measure from final passage.

D.C. Council Chairman Phil Mendelson (D) — who, along with his colleagues, put the measure before voters — said in a statement that he was disappointed with the GAO opinion but that it gave Congress an “opportunity to acknowledge explicitly the right of District residents to determine how their tax dollars should be spent.”

Begich said he is eager to bring legislation to a vote. “The faster we do this, to be frank with you, is one less thing that Congress has to meddle in that we don’t need to meddle in,” he said.

The GAO did not wade into that battle. “In this opinion we express no views on the merit of greater budget autonomy for the District; it is a matter that rests with the Congress,” Poling wrote.

Aaron C. Davis contributed to this report.