(CNN) The Government Accountability Office said the Trump administration broke the law when it withheld US security aid to Ukraine last year that had been appropriated by Congress, a decision that's at the heart of the House's impeachment case against President Donald Trump.

The GAO, a nonpartisan congressional watchdog, said in a decision issued Thursday that the White House budget office violated the Impoundment Control Act, a 1974 law that limits the White House from withholding funds that Congress has appropriated.

The Office of Management and Budget told the GAO it "withheld the funds to ensure that they were not spent 'in a manner that could conflict with the President's foreign policy,'" said Thomas Armstrong, the GAO's general counsel. But the GAO rejected that argument.

"Faithful execution of the law does not permit the President to substitute his own policy priorities for those that Congress has enacted into law," the GAO wrote. "OMB withheld funds for a policy reason, which is not permitted under the Impoundment Control Act. The withholding was not a programmatic delay. Therefore, we conclude that OMB violated the ICA."

The decision will add fuel to the Democratic allegations that Trump's conduct ran afoul of the law when the his administration withheld $400 million in security aid to Ukraine while the President and his team pushed Ukraine to open an investigation into the President's political rivals.

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