Volker is not considered a Trump ally and reportedly felt he was “forced out” by the president’s loyalists.

He is expected to point out the risks created by the freewheeling effort led by presidential lawyer Rudy Giuliani to unearth damaging information about Biden and his son Hunter, CNN reported.

“I think he was doing the best he could,” said retired senior US diplomat Daniel Fried, who described the actions of his former colleague as trying to guide Ukrainians on “how to deal with President Trump under difficult circumstances”.

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Volker’s role, along with US Secretary of State Mike Pompeo’s confirmation that he was also on Trump’s July 25 call, deeply entangles the State Department in the impeachment inquiry now shadowing the White House.

The State Department said Volker has confirmed that he put a Zelensky adviser in contact with Trump’s lawyer Rudy Giuliani, at the Ukraine adviser’s request, and Giuliani has said he was in frequent contact with Volker.

Volker delivered dozens of pages of documents to Congress on Thursday morning ahead of his planned deposition.

It was not immediately clear whether the documents detail Volker’s side of his conflict with Giuliani.

The whistleblower named Volker as someone who sought to “control the damage” from Giuliani efforts, which forced Ukraine’s embattled government to take sides in an American partisan fight or risk losing critically needed defence aid to fight Russian aggression.

“On the side, you had Rudy Giuliani and he’s out there pursuing the personal interests of Donald Trump … and the long-debunked theory about vice president Biden,” Steven Pifer, who served as ambassador to Ukraine under former president Barack Obama, said on CNN. “That detracted from Ambassador Volker’s role of pursuing America’s national interest of supporting Ukraine.”

Giuliani has at times claimed he was acting with the explicit support of US officials.

The former new York mayor flashed his cell phone during a Fox News interview to display what he said were fawning text messages from Volker, who was tasked with arranging meetings between Ukrainian officials and Giuliani.

The spotlight is an unlikely place for Volker, who was brought into the Trump administration by Trump’s first secretary of state, Rex Tillerson, to serve as envoy for Ukraine.

He worked in a volunteer capacity and had retained his job as head of the John McCain Institute for International Leadership at Arizona State University.

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Though his name may not have been known before last week to most Americans, Volker had a long diplomatic career, often working behind the scenes.

He was a principal deputy assistant secretary for European and Eurasian affairs before becoming the US ambassador to Nato in 2008.

In his most recent role as envoy to Ukraine, he spoke openly of US support for Ukrainian sovereignty.

Last year, he criticised the expansion of Russian naval operations and Russia’s resistance to full deployment of a UN peacekeeping mission in eastern Ukraine to monitor the fight against the Russia-backed separatists.

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Pompeo himself mentioned Volker in an appearance in Rome on Wednesday when he confirmed his participation in the call, saying he had been focused on “taking down the threat that Russia poses” in Ukraine and to help the country build its economy.

Fried described Volker as a “dedicated public servant and professional, a problem solver”.

“In all of the years I’ve worked with him, we never had a partisan conversation,” Fried said. “He’s an utter professional.”