Trump has denied any wrongdoing and called the inquiry "the greatest scam in the history of American politics". "And I don't know if I recalled her or somebody recalled her, but I heard very, very bad things about her for a long period of time".
Yovanovitch has led USA embassies in Kyrgyzstan and Armenia and is now a State Department fellow at Georgetown University.
Yovanovitch has always known that the role of diplomat "wasn't about her" but about "serving American national interests and supporting the people around her", said Nancy McEldowney, a former USA ambassador to Bulgaria who now directs a Foreign Service program at Georgetown.
Yovanovitch - who had previously served as ambassador to Kyrgyzstan and Armenia - was scheduled to testify on October 2 but that session was canceled.
A person familiar with the negotiations between the committee and Sondland's legal team said there has been no contact between the ambassador and the White House since Sondland was directed not to testify. A trial on whether to remove him from office would then be held in the U.S. Senate, where the Republicans who control the chamber have shown little appetite for ousting the president. She has dual U.S. -U.K. citizenship. Trump tweeted immediately afterward that he couldn't let Sondland testify because the Democratic-led probe was "compromised". The committees are intelligence, foreign and oversight. The ambassador "respects the Committee's interest in reviewing all relevant materials", the statement explained, "however, federal law and State Department regulations prohibit him from producing documents concerning his officials responsibilities".
In a Friday morning statement, an attorney for Sondland said, "Notwithstanding the State Department's current direction not to testify, Ambassador Sondland will honor the Committees' subpoena, and he looks forward to testifying on Thursday".
Democrats want to ask him about text messages released last week that show him and two other US diplomats acting as intermediaries as Trump urged Ukraine to investigate Ukraine's involvement in the 2016 USA election and Hunter Biden's involvement with a gas company there. House Democrats received a cache of the texts as part of their impeachment inquiry.
"The president has been crystal clear no quid pro quo of any kind", Sondland said.
Sondland flew from Brussels to Washington to voluntarily answer questions about text messages between him and the senior diplomat at the US embassy in Ukraine, Bill Taylor, about whether military assistance was being conditioned on investigations related to political campaigns.
The pair, Ukraine-born Lev Parnas and Belarus-born Igor Fruman, were arrested at an airport outside Washington carrying one-way tickets to Vienna.
Meanwhile, a top aide to Secretary of State Mike Pompeo has resigned, a move that comes amid growing concern at the State Department over the treatment of career staff amid the Ukraine inquiry.
President Trump, despite admitting that he wanted the Bidens investigated, has said he did nothing wrong.