Ambassador Gordon Sondland (C) arrives at the US Capitol October 17, 2019, in Washington, DC.

Gordon Sondland, the U.S. ambassador to the European Union, is set to testify to Congress on Thursday that he understood that President Donald Trump "directed" Rudy Giuliani's participation in efforts to get Ukraine to investigate former Vice President Joe Biden.

Sondland, in prepared remarks to House impeachment investigators, said Giuliani told him that Trump wanted Ukraine to probe both a conspiracy theory that that nation played a role in trying to undercut Trump's presidential candidacy in 2016, and a natural gas firm in Ukraine that had Hunter Biden on its board until earlier this year.

"My understanding was that the president directed Mr. Giuliani's participation, that Mr. Giuliani was expressing the concerns of the president," Sondland said in the prepared remarks at closed hearing.

Sondland said several times in the remarks that he was "disappointed" in Trump's decisions on Ukraine, including having Giuliani — the president's private attorney — take a lead role.

"I did not understand, until much later, that Mr. Giuliani's agenda might have also included an effort to prompt the Ukrainians to investigate Vice President Biden or his son or to involve Ukrainians, directly or indirectly, in the president's 2020 re-election campaign," Sondland wrote in the testimony.

Sondland, a wealthy hotelier, noted that he is "a lifelong Republican," who is "not here to push an agenda."

"I am here to tell the truth," said Sondland, whose originally scheduled appearance at the House last week was blocked by the State Department. The Trump administration has refused to cooperate with the impeachment inquiry.

The House later issued a subpoena for Sondland's testimony, which he said "I am pleased to provide."

Trump and Giuliani claim, without evidence, that Biden's previous call, as vice president, for Ukraine to dismiss a prosecutor in that country was done to protect Hunter Biden from investigation there. The former vice president's pressure on Ukraine to remove the prosecutor was in line with calls from European countries, which were concerned the prosecutor was not tackling corruption in Ukraine.