House Democrats have subpoenaed Gordon Sondland after the Trump administration blocked the US ambassador to the EU from testifying for the impeachment inquiry.

“In light of Secretary Pompeo’s direct intervention to block your appearance before our Committees, we are left with no choice but to compel your appearance at a deposition,” the House democrats’ subpoena reads.

The subpoena comes after the White House declared it will refuse to cooperate with the “baseless, unconstitutional” congressional impeachment inquiry, releasing an extraordinary letter that sets President Donald Trump on a collision course with the Democratic-led US House of Representatives.

“Your unprecedented actions have left the President with no choice,” lawyer Pat Cipollone wrote to Democratic House leaders, in a letter released late on Tuesday (Washington time).

“In order to fulfil his duties to the American people, the constitution, the executive branch and all future occupants of the office of the presidency, President Trump and his administration cannot participate in your partisan and unconstitutional inquiry under these circumstances.”

The letter came shortly after the Trump administration abruptly blocked a key witness in the Ukraine scandal from appearing before a congressional impeachment inquiry.

Mr Sondland had been due to testify on Tuesday (local time) but received a midnight voicemail message from the State Department instructing him not to appear, Democrats said.

House speaker Nancy Pelosi slammed the move as an “abuse of power” but Mr Trump hit back, dismissing the impeachment inquiry process as a “kangaroo court”.

BREAKING: In letter to US House leaders, White House counsel declares that “President Trump cannot permit his Administration to participate in this partisan” impeachment inquiry. pic.twitter.com/qRvZpOHjPp

— NBC News (@NBCNews) October 8, 2019

The letter from Mr Cipollone was the White House’s most aggressive response yet to the impeachment inquiry, which has cast a pall over Mr Trump’s 2020 re-election campaign.

It came as extreme measures are being discussed to protect the whistleblower whose complaint sparked the probe.

Among the measures being considered are disguising the whistleblower’s voice and image and testifying at an off-site location.

Mr Sondland had been due to be grilled over Mr Trump’s push to get Ukraine to investigate his Democrat rival, Joe Biden, ahead of the 2020 election.

Text messages released earlier show Mr Sondland was one of the US officials who were encouraging Ukraine to investigate the energy company that appointed Mr Biden’s son Hunter to its board.

In exchange, the US officials dangled the offer of a Washington meeting with Mr Trump for Ukraine’s new president, Volodymyr Zelenskiy.

Mr Sondland’s lawyer, Robert Luskin, said his client was “profoundly disappointed” that he would not be able to testify before the congressional investigation.

He added that “Ambassador Sondland travelled to Washington from Brussels in order to prepare for his testimony and to be available to answer the Committee’s questions”.

The ambassador had recovered the requested communications relating to the Ukraine “quid pro quo” scandal from his devices and turned them over to the State Department. It is now withholding them from the committees involved in the inquiry.

The chairs of the three investigating committees have accused the Trump administration of “an obstruction of the impeachment inquiry”.

“These actions appear to be part of the White House’s effort to obstruct the impeachment inquiry and to cover up President Trump’s misconduct from Congress and the American people,” Adam Schiff, Eliot Engel and Elijah Cummings said in a joint statement.

“Ambassador Sondland’s testimony and documents are vital, and that is precisely why the administration is now blocking his testimony and withholding his documents.

“We consider this interference to be obstruction of the impeachment inquiry.

Mr Trump has given his version of why the envoy was blocked, tweeting on Tuesday that “I would love to send Ambassador Sondland” to testify, “but unfortunately he would be testifying before a totally compromised kangaroo court”.

I would love to send Ambassador Sondland, a really good man and great American, to testify, but unfortunately he would be testifying before a totally compromised kangaroo court, where Republican’s rights have been taken away, and true facts are not allowed out for the public….

— Donald J. Trump (@realDonaldTrump) October 8, 2019

Mr Sondland was a Seattle-based hotelier until Mr Trump nominated him to his position as ambassador in May.

The House of Representatives foreign affairs, intelligence and oversight committees are expected to ask Mr Sondland to explain why he became involved in dealings with Ukraine, which is not a member of the EU.

-with AAP