“As the sitting U.S. Ambassador to the EU and employee of the State Department, Ambassador Sondland is required to follow the Department’s direction,” his attorney, Robert Luskin, said in a statement. “Ambassador Sondland is profoundly disappointed that he will not be able to testify today. Ambassador Sondland traveled to Washington from Brussels in order to prepare for his testimony and to be available to answer the Committee’s questions.”

The State Department did not immediately respond to a request for comment.

Though Sondland, through his lawyer, expressed hope that he would still be permitted to testify, the move to block his appearance is an indication that the Trump administration is continuing efforts to prevent officials from cooperating with any congressional inquiries. Democrats have already indicated they consider these efforts to be evidence of obstruction.

Trump on Tuesday defended the decision to block Sondland from appearing before House investigators.

"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 to see," Trump tweeted.

Sondland, according to text messages provided to Congress by former U.S. representative to Ukraine Kurt Volker, appeared to be involved in discussions surrounding a potential September White House visit for newly elected Ukrainian President Volodymyr Zelensky — as well as discussions about withholding that visit amid efforts by Trump to press Zelensky to investigate the Bidens for dealings in Ukraine.

In the exchange, Sondland denied the existence of a quid pro quo, but he also said Trump appeared focused on achieving a “deliverable” before agreeing to meet with Zelensky. The text chain also included discussion of whether military aid was being withheld from Ukraine to force Zelensky to comply.

Trump, in his tweets on Tuesday morning, highlighted the text in which Sondland denied an extortion attempt.

"Importantly, Ambassador Sondland’s tweet, which few report, stated, 'I believe you are incorrect about President Trump’s intentions. The President has been crystal clear: no quid pro quo’s of any kind.' That says it ALL!" Trump wrote.

The news that Sondland was being blocked from testifying came a day after two associates of Trump’s personal lawyer Rudy Giuliani — who has spent months urging Ukraine to investigate the Bidens — also indicated they would not appear for scheduled depositions this week.

Democrats say moves to block their investigations by the White House could feed an obstruction of Congress article of impeachment against Trump, and the stonewalling also raises significant obstacles to unearthing new information about the allegations about Trump’s interactions with Ukraine — a matter that has accelerated the House’s drive toward impeachment.

House Intelligence Chairman Adam Schiff repeated that assertion Tuesday morning, speaking to reporters in the Capitol where he denounced the move. Schiff said the committee had been blindsided by Sondland's refusal to appear, telling reporters there were "conversations well into yesterday afternoon and evening with the State Department legal adviser where there was no indication that this ambassador would be a no-show."

The chairman also ripped the State Department for withholding material Sondland had turned over, calling them "deeply relevant" to the impeachment inquiry.

"It is hard to overstate the significance of not just Ambassador Sondland's testimony and documents but the testimony of others as well. 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, a co-equal branch of government," he said.

One of the president's top congressional allies, Rep. Jim Jordan, also expressed disappointment that Sondland would not be testifying, claiming his testimony would have reinforced Trump's narrative of events.

But "we understand the reason why the State Department decided not to have Ambassador Sondland appear today," he said, before slamming Schiff. "It's based on the unfair and partisan process that Mr. Schiff has been running."

Nahal Toosi, Anita Kumar and Caitlin Oprysko contributed to this report.