The White House broke the law by withholding aid to Ukraine that had been approved by the US Congress, a government watchdog has said.
The Government Accountability Office (GAO) ruling is a potential blow to President Donald Trump as he faces an impeachment trial in the Senate.
He is accused of freezing aid to pressure Ukraine to investigate a political rival.
Ukraine has opened a probe on separate allegations linked to the impeachment.
Why was the aid freeze illegal?
"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 decision by the GAO said.
The White House Office of Management and Budget (OMB) "withheld funds for a policy reason, which is not permitted under the Impoundment Control Act (ICA)", the ruling continued.
The White House said it disagreed with the GAO's opinion, accusing the agency of trying to "insert themselves into the media's controversy of the day".
Democrats welcomed the ruling.
In a news conference on Thursday morning, House of Representatives Speaker Nancy Pelosi said the White House "broke the law".
What led to Ukraine's investigation?
The ruling comes as Ukrainian authorities began a criminal investigation into whether the former US Ambassador to Ukraine, Marie Yovanovitch, was spied on.
The former envoy's movements were being monitored, according to letters, phone records, notes and flash drives obtained from Lev Parnas, a Ukrainian-American businessman.
Mr Parnas is an aide to Mr Trump's personal lawyer Rudy Giuliani.
Some of the materials – which House Democrats are presenting as evidence in the impeachment inquiry – show Mr Parnas and Mr Giuliani discussing the removal of Ms Yovanovitch, who was fired last May for reasons that remain unclear.
Several text messages from a Republican congressional candidate, Robert Hyde, to Mr Parnas appear to suggest the ambassador was being tracked in the capital Kyiv.
Mr Parnas was given apparent updates from Mr Hyde on the ambassador's location and mobile phone.
But Mr Parnas told MSNBC on Wednesday that he did not think Mr Hyde's surveillance talk was credible.
"He was either drunk," said Mr Parnas, "or he was trying to make himself bigger than it was, so I didn't take it seriously".
Mr Parnas said Ms Yovanovitch was removed because she was in the way of a Trump-approved plan to prod Ukraine to announce an investigation into former Vice-President Joe Biden.
Mr Biden is a potential Democratic rival to Mr Trump in the White House election this November.
Ms Yovanovitch has called for an investigation into the messages, which her lawyer called "disturbing".
Mr Parnas told MSNBC that he did not think Mr Hyde's talk about surveillance was credible.
"He was either drunk," said Mr Parnas, "or he was trying to make himself bigger than it was, so I didn't take it seriously".
What else did Lev Parnas say?
Mr Parnas told NBC that he was in Ukraine to put pressure on officials to investigate Mr Biden and his son, Hunter, on behalf of Mr Trump and Mr Giuliani.
He said that President Trump "knew exactly what was going on".
"I wouldn't do anything without the consent of Rudy Giuliani or the president. Why would [Ukrainian] President Zelensky's inner circle or [Interior] Minister [Arsen] Avakov or all these people or [former] President [Petro] Poroshenko meet with me?
"Who am I? They were told to meet with me. And that's the secret that they're trying to keep. I was on the ground doing their work," he added.
Documents show that Mr Parnas was in regular contact with Mr Giuliani as well as Ukrainian officials.
The files also indicate Mr Parnas was directly involved in trying to have President Zelensky announce an investigation into Mr Biden.
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