Gordon Sondland, the US ambassador to the EU, was set to be first Trump-appointee questioned in inquiry.

The Trump administration on Tuesday blocked an ambassador from testifying to the United States House of Representatives impeachment investigation of President Donald Trump‘s dealings with Ukraine, a move top House Democrats promised to counter with a subpoena.

The chairmen of three House committees leading the impeachment investigation said they would compel testimony from US Ambassador to the European Union Gordon Sondland, a Trump donor who started his diplomatic job in July. The investigators are interested in what the ambassador knew and what his role was in Trump’s efforts to get Ukraine to investigate former Vice President Joe Biden, who leads the field of potential challengers to the Republican president.

Through his lawyer on Tuesday, Sondland said he hoped “the issues raised by the State Department that preclude his testimony will be resolved promptly”.

“He stands ready to testify on short notice, whenever he is permitted to appear,” Sondland’s lawyer, Robert Luskin, said in a statement.

State Department officials could not immediately be reached to comment on what issues had blocked his testimony.

The impeachment inquiry is focused on a whistle-blower’s allegations that Trump used nearly $400m in US military aid to secure a promise from Ukraine’s president to investigate Biden, a leading candidate for the 2020 Democratic presidential nomination, and his son. There has been no evidence of wrongdoing by the Bidens. Trump has rejected the allegations.

The whistle-blower’s legal team was in the final stages of talks for the intelligence officer to speak to Democratic and Republican-led congressional intelligence committees as early as this week, congressional officials said.

Representative Adam Schiff, the Democratic chairman of the House intelligence committee spearheading the impeachment inquiry, told reporters the move to block Sondland from testifying or turning over his text messages or emails was obstruction.

“We will consider this act today … to be further acts of obstruction of a coequal branch of government,” Schiff told reporters.

{articleGUID}

Sondland was considered a key witness and members of the committees were expected to ask him why he became involved in dealings with Ukraine, which is not a member of the EU. Sondland had previously said on Ukrainian television that Trump had given him a “special assignment” to oversee relations between the two countries, the New York Times reported.

Luskin said that Sondland “believes strongly that he acted at all times in the best interests of the United States”.

The president, shortly after the announcement was made public, tweeted that he would “love” to have sent Sondland to sit for a Capitol Hill deposition.

“But unfortunately he would be testifying before a totally compromised kangaroo court, where Republican’s rights have been taken away, and true facts are not allowed out for the public to see,” the president said.

According to text messages released by House committee leaders last week, Sondland was heavily involved in contacts with Ukrainian President Volodymyr Zelensky and his adviser, before and after the phone call, as the Ukrainian leader sought a meeting with Trump.

The messages showed that Sondland stressed that such a meeting hinged on Ukraine’s commitment to investigating a theory pushed by Trump that Ukraine was involved in 2016 election meddling, as well as investigating the Burisma gas company, where Biden’s son, Hunter, served on the board.

In one of the text messages, for example, Sondland emphasised that Trump “really wants the deliverable”, an apparent reference to the investigation into the Bidens.

The request for Sondland’s appearance also marked a shift in the investigation, as he would have been the first Trump political appointee to answer questions before the committees. Previous witnesses have been career officials, including the former US special envoy for Ukraine, Kurt Volker, and Michael Atkinson, the inspector general of the US Intelligence Community.

Volker, one of the diplomats involved in the text exchanges, resigned from his part-time role before his testimony last week.

Another career diplomat, Marie Yovanovitch, is scheduled to meet with the committees behind closed doors on Friday. Yovanovitch was the US ambassador to Ukraine until Trump recalled her in May before her term was up, after the president’s supporters questioned her loyalty.

Secretary of State Mike Pompeo, who heads the State Department, had previously accused House Democrats of attempting to “intimidate” and “bully” staff members by calling on them to testify under oath.

Pompeo let a deadline for the House subpoena for documents relating to Ukraine to expire on Friday. The House has given a deadline of October 15 for the White House, the White House’s budget office and the Defense Department to comply with similar subpoenas.