Ambassador to the European Union Gordon Sondland had voluntarily agreed to be deposed by House Democrats .
The White House on Tuesday blocked a top US diplomat from speaking to Congress as part of the impeachment investigation of Donald Trump, dramatically escalating the battle with Democrats over his presidency.
In response, Democrats heading the impeachment probe announced they will subpoena Ambassador to the European Union Gordon Sondland for his testimony and documents.
Mr Sondland is enmeshed in the scandal over how Trump tried to pressure Ukraine into investigating his political rival Joe Biden.
The diplomat's lawyer said he had agreed to be deposed by House Democrats probing the matter. But "early this morning, the US Department of State directed... Sondland not to appear today for his scheduled transcribed interview," his attorney Robert Luskin said in a statement.
Mr Trump swiftly took responsibility for the decision, taking to Twitter to say he would "love" to have sent Mr Sondland to sit for a Capitol Hill deposition.
"But unfortunately he would be testifying before a totally compromised kangaroo court, where Republicans' rights have been taken away, and true facts are not allowed out for the public to see," the president wrote.
The order not to testify - made in a midnight call by the State Department to Sondland hours before his scheduled appearance - intensifies the confrontation between the White House and Democrats investigating the president for possible impeachable offences including obstruction of justice.
Democrats have delivered subpoenas to multiple administration officials, including Secretary of State Mike Pompeo, ordering them to turn over relevant documents.
The White House has largely refused to cooperate with the probe, which House Speaker Nancy Pelosi formally launched last month after revelations that Trump pressured Ukraine's President Volodymyr Zelensky in a July 25 phone call.
Mr Sondland is a wealthy hotelier who was a major donor to Trump's 2016 presidential campaign.
He was one of a handful of US diplomats on a text message chain between July and September, provided to Democrats leading the probe, that go to the heart of the investigation's focus.
The messages between the diplomats, Mr Trump's personal lawyer Rudy Giuliani and a Ukrainian presidential aide show that they helped coordinate administration efforts to pressure Ukraine into investigating former vice president Biden, a 2020 White House hopeful, as the country sought US military aid and access.
In their texts, Mr Sondland and other diplomats discussed leveraging a potential summit between Mr Trump and Mr Zelensky on a pledge to investigate 2016 election interference and a Ukrainian company that employed Biden's son Hunter.
'Evidence of obstruction'
The impeachment investigation's conductors warned that the White House's silencing of Mr Sondland only contributes to Democrats' arguments that a comprehensive effort of illicit obstruction is underway.
House Intelligence Committee chairman Adam Schiff said investigators have learned that Mr Sondland has text messages or emails on a personal device that are "deeply relevant" to the probe.
Mr Schiff has requested those communications, but "the State Department is withholding those messages as well," he said.
"The failure to produce this witness, the failure to produce these documents, we consider yet additional strong evidence of obstruction of the constitutional functions of Congress," Mr Schiff told reporters.
In one text message to a diplomat who expressed concern that the administration was withholding nearly $400 million in military aid to Ukraine "for help with a political campaign," Mr Sondland insisted that Mr Trump had been "crystal clear, no quid pro quo's of any kind" were demanded.
"I suggest we stop the back and forth by text,"Mr Sondland added.
Mr Sondland was prepared to testify, and "believes strongly that he acted at all times in the best interests of the United States," his lawyer said.
Republicans who back Trump's call for Ukraine and China to launch investigations of the Bidens, under the guise of rooting out corruption, have launched a counter-offensive.
Senate Judiciary Committee chairman Lindsey Graham announced Monday that he will invite Giuliani, who has acknowledged soliciting Ukraine's help in investigating Mr Biden, to testify before the panel as part of a new Senate anti-corruption inquiry involving Ukraine.
"Good. I have questions," tweeted Senator Kamala Harris, a 2020 presidential candidate.
Soliciting foreign help in a US election is illegal, whether or not inducements are offered.