WASHINGTON (AFP) - The US ambassador to the European Union will testify to Congress next week as part of the impeachment investigation of President Donald Trump, despite a State Department order not to, his lawyer said on Friday (Oct 11).

Gordon Sondland, a wealthy donor to Trump's 2016 campaign who was part of text message chains discussing the president's effort to pressure Ukraine to investigate a political rival, was scheduled to testify this week but the administration blocked him.

That prompted a subpoena by committees leading the probe in the Democratic-controlled House of Representatives for Sondland to be deposed in closed session.

"Notwithstanding the State Department's current direction to not testify, Ambassador Sondland will honour the Committees' subpoena, and he looks forward to testifying on Thursday," Sondland's lawyer Robert Luskin said.

The statement was released on Friday on the same morning that the former US ambassador to Kiev, Marie Yovanovitch, was to testify before the panels about what she knows of the Trump administration's pressure effort on Ukraine.

House Democrats are looking into whether Yovanovitch, a career diplomat, was removed by Trump because she failed to go along with a bid coordinated by Trump's personal attorney Rudy Giuliani to get Ukraine to open an investigation into Democratic presidential hopeful Joe Biden and his son.

Even one hour before her scheduled testimony, it was not clear if Yovanovitch would appear or be blocked by the administration.