The Trump administration violated the law by withholding $391 million in military assistance to Ukraine, the US government's independent auditor has said.

Government Accountability Office general counsel Thomas Armstrong laid out the agency's conclusion that the president and his budget officials violated the law by withholding $391 million in security assistance to Ukraine in a decision memorandum released on Thursday. Although it is technically considered part of the legislative branch, the GAO headed by the Comptroller General of the United States, who is nominated by the president and confirmed by the Senate to a 15-year, non-renewable term. The agency was established as the General Accounting Office in 1922, and is widely respected for its independence and record of ensuring proper use of taxpayer funds.

"Faithful execution of the law does not permit the president to substitute his own policy priorities for those that Congress has enacted into law," Mr Armstrong wrote.

Specifically, Mr Armstrong concluded that top officials with White House Office of Management and Budget had violated a 1974 law prohibiting the executive branch from not spending money Congress had appropriated, because OMB officials had withheld the funds for an impermissible "policy reason".

That law, known the Impoundment Control Act, was enacted in response to then-president Richard Nixon's refusal to spend funds appropriated by the legislative branch. It allows the president to specifically request that Congress rescind a particular appropriation if he or she does not want to spend those funds, but it requires that the funds be spent as appropriated if Congress takes no action within 45 days.

Trump impeachment: Who's who in the Ukraine scandal Show all 26 1 /26 Trump impeachment: Who's who in the Ukraine scandal Trump impeachment: Who's who in the Ukraine scandal Donald Trump Accused of abusing his office by pressing the Ukrainian president in a July phone call to help dig up dirt on Joe Biden, who may be his Democratic rival in the 2020 election. He also believes that Hillary Clinton’s deleted emails - a key factor in the 2016 election - may be in Ukraine, although it is not clear why. EPA Trump impeachment: Who's who in the Ukraine scandal The Whistleblower Believed to be a CIA agent who spent time at the White House, his complaint was largely based on second and third-hand accounts from worried White House staff. Although this is not unusual for such complaints, Trump and his supporters have seized on it to imply that his information is not reliable. Expected to give evidence to Congress voluntarily and in secret. 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AP Trump impeachment: Who's who in the Ukraine scandal Rick Perry Trump reportedly told a meeting of Republicans that he made the controversial call to the Ukrainian president at the urging of his own energy secretary, Rick Perry, and that he didn’t even want to. The president apparently said that Perry wanted him to talk about liquefied natural gas - although there is no mention of it in the partial transcript of the phone call released by the White House. It is thought that Perry will step down from his role at the end of the year. Getty Trump impeachment: Who's who in the Ukraine scandal Joe Biden The former vice-president is one of the frontrunners to win the Democratic nomination, which would make him Trump’s opponent in the 2020 election. Trump says that Biden pressured Ukraine to sack a prosecutor who was investigating an energy company that Biden’s son Hunter was on the board of, refusing to release US aid until this was done. 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And here we are.” AP Trump impeachment: Who's who in the Ukraine scandal David Holmes The Ukraine-based diplomat described being in a restaurant in Kiev with Gordon Sondland while the latter phoned Donald Trump. Holmes said he could hear the president on the other end of the line – because his voice was so “loud and distinctive” and because Sondland had to hold the phone away from his ear – asking about the “investigations” and whether the Ukrainian president would cooperate. REUTERS

The White House's failure to abide by the Impoundment Control Act was noted by Defence Department comptroller Elaine McCusker in a series of emails, in which Ms McCusker repeatedly informed OMB associate director of national security programmes Michael Duffey that continuing to withhold funds meant for the Pentagon's Ukraine Security Assistance Initiative would require a "recission notice" from the president to Congress.

The emails revealed that Mr Duffey told Ms McCusker that the decision to place a hold on the Ukraine funds came as a "clear direction" from president Donald Trump.

Mr Duffey first directed Pentagon officials to begin withholding the funds just hours after the president's 25 July phone call with Ukrainian president Volodymyr Zelensky, in which Mr Trump responded to his Ukrainian counterpart's request to purchase Javelin anti-tank missiles with his own request for "a favour" consisting of investigations into former vice president Joe Biden and a debunked conspiracy which alleges that Ukraine -- not Russia -- interfered in the 2016 election.

While the president and his allies have argued that Mr Trump's decision to withhold the funds was a proper use of his constitutional authority to conduct foreign policy, Mr Armstrong flatly rejected such a defence.

"An appropriations act is a law like any other; therefore, unless Congress has enacted a law providing otherwise, the President must take care to ensure that appropriations are prudently obligated during their period of availability," he wrote, adding later that the US constitution "grants the President no unilateral authority to withhold funds from obligation".

The senator who initially requested the GAO's investigation, Maryland Democrat Chris Van Hollen, said in a statement that the agency's "bombshell legal opinion...demonstrates, without a doubt, that the Trump Administration illegally withheld security assistance from Ukraine".

"This violation of the law reflects a contempt for the Constitution and was a key part of his corrupt scheme to abuse the power of the presidency for his personal political purposes. The GAO's independent findings reinforce the need for the Senate to obtain all relevant documents and hear from key fact witnesses in order to have a fair trial," he said.

Mr Trump's attempt to withhold security assistance to Ukraine in order to strong-arm Mr Zelensky into announcing investigations into his political rival is at the centre of the abuse of power allegations for which the US House of Representatives voted to impeach him on 18 December, making him just the third American president to face a trial before the US Senate with the possibility of being removed from office.