US President Donald Trump has been warned that he is “not above the law” after the White House refused to cooperate with an impeachment inquiry against him.

In a letter to top Democrats last night, lawyers for President Trump described the investigation into his dealings with Ukraine as “constitutionally invalid.”

The White House said President Trump’s and all members of his Executive Branch would refuse to participate in the inquiry.

"Unlawful attempt to hide the facts"

In response, the House Speaker Nancy Pelosi said the letter was simply the latest attempt to cover President Trump’s “betrayal of our democracy, and to insist that the President is above the law.”

“This letter is manifestly wrong and is simply another unlawful attempt to hide the facts of the Trump Administration’s brazen efforts to pressure foreign powers to intervene in the 2020 elections,” she wrote.

“Despite the White House’s stonewalling, we see a growing body of evidence that shows that President Trump abused his office and violated his oath to ‘protect, preserve and defend the Constitution.’

“The White House should be warned that continued efforts to hide the truth of the President’s abuse of power from the American people will be regarded as further evidence of obstruction.

“Mr President, you are not above the law.  You will be held accountable.”

House Speaker Nancy Pelosi speaks with the media at Harborview Medical Center in Seattle, 08-10-2019. Image: AP Photo/Elaine Thompson

Election interference

The investigation focuses on whistleblower claims that President Trump used his position to pressure Ukrainian President Volodymyr Zelenskiy into investigating Democratic presidential hopeful Joe Biden.

President Trump is accused of using a phone call in July to ask the Ukrainian President to investigate the business dealings of Mr Biden’s son.

US aid to Ukraine was withheld before the phone call.

"Partisan and unconstitutional"

In their letter, lawyers for President Trump called the inquiry “highly partisan and unconstitutional” and claimed it threatened “grave and lasting damage” to US democracy.

“Put simply, you seek to overturn the results of the 2016 election and deprive the American people of the President they have freely chosen,” it reads.

“Many Democrats now apparently view impeachment not only as a means to undo the democratic results of the last election, but as a strategy to influence the next election – which is barely more than a year away.”

President Trump last night blocked his EU ambassador from giving evidence to the inquiry.