The Trump administration has blocked the US ambassador to the European Union from appearing before a congressional impeachment investigation.
Gordon Sondland was due to meet behind closed doors on Tuesday with staff from three Democratic-led House committees.
But the envoy was directed by the Department of State not to attend.
Mr Sondland was set to be grilled about any role he played in prodding Ukraine to investigate President Donald Trump’s political rival, Joe Biden.
The Democratic-led inquiry is trying to establish whether the Republican president withheld nearly $400m (£327m) in aid to nudge Ukraine’s president into launching an inquiry into Mr Biden, whose son, Hunter Biden, was on the board of a Ukrainian energy company.
In a phone call on 25 July, Mr Trump asked the newly elected Volodymyr Zelensky to scrutinise the former US vice-president, who is a leading Democratic contender for next year’s White House election.
A whistleblower raised concerns about the phone call, and the Democratic leader of the House of Representatives, Speaker Nancy Pelosi, announced a formal impeachment investigation last month.
Impeachment is a measure by which Congress can seek to remove a president from office.