Gordon Sondland, the US's ambassador to the European Union, will turn on President Donald Trump and his lawyer Rudy Giuliani when he testifies in Congress' impeachment inquiry on Thursday.

Sondland will tell lawmakers he was against Trump's decision to conduct Ukraine policy through Giuliani, according to his opening statement.

He will also say he didn't know "until much later" that Giuliani wanted Ukraine to investigate Burisma Holdings, the natural-gas company whose board Hunter Biden sat on, to get dirt on former Vice President Joe Biden.

The ambassador is also expected to criticize Trump's decision to hold up a $400 million military-aid package to Ukraine over the summer. It "should not have been delayed for any reason," Sondland's opening statement said.

But Sondland will likely face significant pushback from lawmakers on some of his claims that have been contradicted by media reporting and testimony from other officials.

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Gordon Sondland, the US's ambassador to the European Union, turned on President Donald Trump and his lawyer Rudy Giuliani in his opening statement to Congress on Thursday.

Sondland is testifying as part of House Democrats' formal impeachment inquiry looking at whether Trump used his public office for private gain.

Sondland is one of a trio of diplomats — with Kurt Volker, the former special representative for Ukraine, and Bill Taylor, the US's chief diplomat in Ukraine — whose explosive text messages to one another revealed just how intricately involved US government officials were in Giuliani's pressure campaign on Ukraine.

Sondland will tell lawmakers that he staunchly opposed Trump's request to conduct Ukraine policy through Giuliani. Trump and Giuliani have been working since at least last spring to urge the Ukrainian government to investigate corruption allegations against former Vice President Joe Biden and his son Hunter and to help discredit the FBI's finding that Russia interfered in the 2016 US election.

Read more: The FBI's investigation of Rudy Giuliani includes a counterintelligence aspect that suggests he may be a national security threat

At the heart of those efforts is a July 25 phone call during which Trump repeatedly pressured Ukrainian President Volodymyr Zelensky to accede to his demands. Days before the call, Trump held up a nearly $400 million military-aid package to Ukraine. During their conversation, Trump reminded Zelensky that "we do a lot for Ukraine" and followed up by asking Zelensky to "do us a favor though" and investigate Joe Biden and the origins of the Russia investigation.

Sondland and Volker were instrumental in conveying Trump's demands to Zelensky before and after the call.

In addition to establishing a channel of communication between Giuliani and senior Ukrainian government officials, Volker and Sondland furthered the impression that a good relationship between Trump and Zelensky — and a White House meeting between the two — was conditional on Zelensky helping the US president's political agenda.

On Thursday, Sondland will tell lawmakers he was "disappointed by the president's direction that we involve Mr. Giuliani," his opening statement said.

He will also testify that he got in touch with Giuliani at Trump's direction anyway and that Giuliani indicated that a White House visit for Zelensky was predicated on Ukraine investigating Burisma Holdings, a Ukrainian natural-gas company whose board Hunter Biden sat on, and the Russia probe.

Read more: Trump's presidency is disintegrating as he faces his worst 30 days since taking office

Sondland's opening statement said that "Giuliani specifically mentioned the 2016 election (including the DNC server) and Burisma as two anti-corruption investigatory topics of importance for the president."