Ambassador Yovanovitch / Photo frmo UNIAN

The former U.S. ambassador to Ukraine was removed from her post after insisting that Rudy Giuliani's requests to Ukrainian officials for investigations be relayed through official channels, according to a former diplomat who has spoken with her.

Yovanovitch was recalled from Kyiv in May as Giuliani – who is Trump's personal attorney and has no official role in the U.S. government – pushed Ukrainian officials to investigate baseless corruption allegations against the Bidens, The Associated Press (AP) reported.

The ambassador, Marie Yovanovitch, is scheduled to testify before congressional lawmakers on Friday as part of the House impeachment inquiry into President Donald Trump.

Read alsoFormer Ukraine envoy scheduled to testify in Trump impeachment probe – media

Democrats say they expect her to appear despite the White House's position that no administration officials cooperate with the probe.

In a July 25 call, Trump told Ukrainian President Volodymyr Zelensky that Yovanovitch was "bad news," according to a partial transcript released by the White House.

Neither Giuliani nor Trump have specified their objections. But a former diplomat, recalling a recent conversation with Yovanovitch, said she was removed after insisting that a request for Ukrainian officials to join in an investigation be relayed according to long-established protocol.

The former diplomat said Yovanovitch refused to do "all this offline, personal, informal stuff" and made clear that the U.S. government had formal ways to request foreign governments' help with investigations.

The former diplomat insisted on anonymity to disclose the private conversation.

The State Department traditionally relies on mutual legal assistance treaties, under which U.S. and foreign officials agree to exchange evidence and information in criminal investigations.

Yovanovitch is scheduled to speak to the House Intelligence, Foreign Affairs and Oversight and Reform committees behind closed doors Friday as part of the impeachment investigation. Despite Trump's assertion that his administration will not cooperate, three people familiar with the deposition said that Yovanovitch is expected to appear. The people requested anonymity to discuss the closed-door meeting.