The Government Accountability Office reaffirmed its opinion Tuesday that “pocket rescissions” are illegal, dealing a modest blow to President Donald Trump’s announcement last week of plans to cancel $810 million in spending without the approval of Congress.

In a letter to congressional leaders, the GAO —the investigative arm of Congress — doubled down on its position that the administration would violate a 1974 budget law by trying to rescind, or cancel, funding that lawmakers had already appropriated in the closing days of the fiscal year, which ends Wednesday. That short timeline gives Congress virtually no time to act on the request as the funding is set to expire.

“Any withholding of appropriated funds beyond their date of expiration, regardless of size, subverts both the constitutional process for enacting federal law and Congress’s constitutional power of the purse,” the GAO wrote. “The President may not force the expiration of budget authority Congress has already enacted and did not rescind.”

The GAO opinion follows Trump’s submittal of a “special message” to Congress Friday laying out plans to cancel $810 million in funding for health, education and other programs. 

Related: White House cancels health, education funds approved by Congress

Under the 1974 Impoundment Control Act, the president may send a request to Congress to cancel funding and withhold the funding for up to 45 days while lawmakers consider the request. Congress may approve the request, reject it or amend it. With the House on recess until after the elections, there is no practical way for the House to consider the request before the fiscal year ends.

But the opinion is unlikely to have any practical effect unless the GAO takes the administration to court. White House officials have said they do not feel bound by the GAO, which is a legislative agency outside the purview of the executive branch.

And White House budget director Russ Vought immediately dismissed the opinion in a post on X, while accusing the GAO of acting with partisan, political motive.

“GAO has become hyper partisan and Congress recently attempted to cut their spending because of it,” Vought wrote. “They make their legal assessments based on whether Democrats are in the White House.”

The pocket rescission has drawn condemnation from Democrats and some Republicans, including Senate Appropriations Chair Susan Collins, R-Maine, who have vowed to fight what they called an illegal action.

Related: Appropriators seek ways to curb Trump’s spending cancellations

The White House argues that pocket rescissions, where funding would expire before Congress takes action, are allowed under the 1974 law because they are not expressly prohibited. But the GAO rejected that argument.

“The plain language of the [budget law] provides that amounts proposed for rescission must be made available for obligation unless Congress completes action on a bill rescinding them,” the GAO said.

The Trump administration used a similar maneuver last year to cancel more than $4 billion in foreign aid without approval from Congress. That case went to the Supreme Court, which allowed the funding to be canceled.

In its brief order, the Supreme Court said the administration “made a sufficient showing” that the law did not allow for the lawsuit, adding that “the asserted harms to the Executive’s conduct of foreign affairs appear to outweigh the potential harm faced by respondents.”

However, the court also said the order “should not be read as a final determination on the merits,” leaving the door open for further rulings in the future.