Public impeachment hearings will begin next week, it has been announced in Congress, marking a new phase in the investigation into Donald Trump’s effort to compel Ukraine to investigate his political opponents.

The chair of the House intelligence committee, Adam Schiff, said that three US diplomats would testify on their account of a shadow foreign policy, orchestrated by Trump and his personal lawyer, Rudy Giuliani, aimed at using military aid and the prospect of a White House visit to convince the government in Kyiv to implicate the former vice-president Joe Biden and his son in corruption investigations.

The acting ambassador to Ukraine, William Taylor, and the deputy assistant secretary for Europe and Eurasia, George Kent, will testify on Wednesday. Marie Yovanovitch, the former ambassador to Kyiv ousted in May on Trump’s orders, will speak on Friday, Schiff said.

“Those open hearings will be an opportunity for the American people to evaluate the witnesses for themselves, to make their own determinations about the credibility of the witnesses, but also to learn firsthand about the facts of the president’s misconduct,” Schiff told reporters.

He also said that the committee would release a full transcript of Taylor’s closed-session testimony.

All the state department witnesses, who have defied administration orders not to cooperate with the investigation, have corroborated a whistleblower report about an attempt by Trump and his circle to pressure the Ukrainian president, Volodymyr Zelenskiy, into announcing specific investigations.

David Hale, the third most senior official at the state department, began his testimony at congressional impeachment hearings on Wednesday. He will reportedly seek to explain the agency’s failure to defend the ambassador to Ukraine as an act of realpolitik.

Hale, the under secretary of state for political affairs, is expected to argue that mounting a robust defence of Yovanovitch in May, when she was removed at Trump’s insistence, would have hurt the state department’s efforts to lift a White House freeze on military aid to Ukraine, according to the Associated Press.

Three other witnesses summoned by the House committees were not expected to appear. The state department counsellor, Ulrich Brechbuhl, a close associate of Mike Pompeo, was on a plane to Germany with the secretary of state, who is attending events commemorating the fall of the Berlin Wall. Russ Vought, the acting director of the office of management and budget, and the energy secretary, Rick Perry, have abided by a White House order not to cooperate.

The congressional committees are examining whether the president abused his office by seeking to use leverage on Ukraine to investigate his political opponents. Part of the inquiry concerns the removal of Yovanovitch from her ambassadorial post, after she refused to take part in Giuliani’s effort to implicate the former vice-president Joe Biden and his son in corruption.

According to the AP, Hale is expected to say that Pompeo and the state department did not protect Yovanovitch for fear of angering Giuliani, a private citizen who had conducted a lobbying campaign against the ambassador. Several witnesses have also testified that security assistance to Ukraine was suspended in July as part of a pressure campaign on Kyiv to investigate the energy firm that hired Biden’s son, Hunter.

Hale was reportedly planning to tell investigators that the state department’s leadership decided that battling Giuliani and the White House over Yovanovitch would expend political capital it needed to persuade Trump to restore the military aid.

The US ambassador to the European Union, Gordon Sondland, amended his original testimony to Congress, to confirm that the military assistance was made conditional on Ukrainian investigations, after initially claiming he was unaware of the quid pro quo.

Gordon Sondland was a perfect fall guy, until he decided to tell the truth | Richard Wolffe Read more

Sondland was accused on Wednesday of making up meetings and conversations. The lawyer acting for the former White House Russia specialist Fiona Hill has disputed his account of meeting Hill for coffee in July just before she was due to leave her national security council job.

In his testimony, Sondland said: “She was pretty upset about her role in the administration, about her superiors, about the president. She was sort of shaking. She was pretty mad.”

Hill’s lawyer, Lee Wolosky, said in a tweet on Wednesday: “Sondland has fabricated communications with Dr Hill, none of which were over coffee.”

“Dr Hill told Sondland what she told lawmakers – the lack of coordination on Ukraine was disastrous, and the circumstances of the dismissal of Amb Yovanovitch shameful,” Wolosky said.

Sondland’s lawyers said they had no comment on Wolosky’s allegation.