The Trump administration violated the law in withholding security assistance to Ukraine, a non-partisan federal watchdog agency said on Thursday, weighing in on a decision by President Trump that is at the heart of the impeachment case against him.

The Government Accountability Office said the White House’s Office of Management and Budget violated the Impoundment Control Act when it withheld nearly $400 million for “a policy reason”, even though the funds had been allocated by Congress.

The decision was directed by the President himself, and during the House impeachment inquiry, administration officials testified that they had raised concerns about its legality to no avail.

“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. “The withholding was not a programmatic delay.”

The impoundment law limits a President’s power to withhold money that has been allocated by Congress, requiring that he secure approval by the legislative branch if he wishes to do so.

The White House budget office promptly rejected the report’s conclusions.

“We disagree with GAO’s opinion,” said Rachel Semmel, a spokeswoman for the budget office. “OMB uses its apportionment authority to ensure taxpayer dollars are properly spent consistent with the president’s priorities and with the law.”

The report, on its own, does not result in any action, although its release just as Trump’s impeachment trial is getting underway is certain to fuel additional questions about the impact of his actions. At the core of the impeachment charges is the allegation that the President withheld the security assistance as part of a campaign to pressure Ukraine to launch investigations of his political rivals that would discredit them and help him win re-election in 2020.

Senator Chris Van Hollen, Democrat of Maryland, a vocal critic of Trump’s decision to block the funds, on Thursday called the GAO report a “bombshell legal opinion”. It “demonstrates, without a doubt, that the Trump Administration illegally withheld assistance from Ukraine and the public evidence shows that the president himself ordered this illegal act,” he wrote on Twitter.

Ukraine investigation

The police in Ukraine have opened a criminal investigation into whether allies of Trump had the US ambassador to the country under surveillance while she was stationed in Kiev, the Ukrainian government said on Thursday.

The move was a remarkable departure from past practice for the government. President Volodymyr Zelensky and his government have worked hard to appear neutral.

But developments have prompted a change of course. On Tuesday, just before Trump’s impeachment trial in the Senate was scheduled to begin, Democrats in the House of Representatives o published text messages to and from Lev Parnas — an associate of Rudolph W. Giuliani — pointing to surveillance of the ambassador, Marie Yovanovitch.