(CNN) Bill Taylor, the top US diplomat in Ukraine and key witness in public House impeachment hearings of President Donald Trump, will depart from his temporary post on January 2, two sources familiar with his plans tell CNN.

The former ambassador had been brought out of retirement earlier this year to take on duties in the embassy after Ambassador Marie Yovanovitch was abruptly recalled to Washington at the President's direction.

As a pivotal witness in the public impeachment hearings, Taylor provided a damning account of Trump telling his appointees to establish a quid pro quo with Ukraine, offering much-needed US military aid and an Oval Office meeting in exchange for personal political favors.

Taylor's appointment expires in early January, the sources said. There are ways the State Department could have tried to extend his stay in Kiev, such as giving him a different title, but there was no effort to do so.

'A snake pit'

The State Department did not reply to a request for comment on Taylor's tenure, or explain who would follow him as top US diplomat in Ukraine.

Taylor testified about his concerns over taking the post in Kiev in the wake of Yovanovitch's unexpected removal in May after a smear campaign led by the President's personal lawyer, Rudy Giuliani.

"I was concerned that there was, I think I put it, a snake pit in Kiev and a snake pit here, and I was not sure that I could usefully serve in that context," Taylor said, according to the transcript.

He had served as the US ambassador to Ukraine from 2006 to 2009, but at the time he was asked to take over at the embassy, he had retired from the Foreign Service and was serving as executive vice president at the US Institute of Peace, a nonpartisan think tank.

Secretary of State Mike Pompeo had to convince him to take the job.

A vacuum

Trump has repeatedly made the unfounded claim that Taylor was a "Never Trumper," first leveling the charge in an October tweet. The President repeated it to a gaggle of reporters on the White House lawn. "Here's the problem," Trump said, referring to Taylor. "He's a Never Trumper."

Taylor has been widely described as a respected and apolitical public servant who has served in both Republican and Democratic administrations.

His departure will create a vacuum in the Trump administration's leadership on US-Ukraine relations. Kurt Volker, the former special representative to Ukraine, left his post earlier this year when the controversy began. And those who are still tending to the relationship do not have the political vote of confidence from the Trump administration.

"They have said we have the existing people with George Kent, Phil Reeker and David Hale to cover the relationship, but the reality is that's not true, because they cannot dedicate the attention to it and the focus," said a source who has worked with all of them on Ukraine policy. "And the fact that they provided testimony [as part of the impeachment inquiry] has put a blanket over their willingness to be outspoken."