Former Federal Bureau of Investigations (FBI) director James Comey accused US President Donald Trump of asking him to drop an investigation of former national security adviser Michael Flynn which was part of a probe into Russia's alleged meddling in the 2016 presidential election.
In dramatic written testimony, Comey said on Wednesday that Trump told him at a meeting in the White House in February, "'I hope you can see your way clear to letting this go, to letting Flynn go.'"
Comey described the exchange as "very concerning".
"I had understood the president to be requesting that we drop any investigation of Flynn in connection with his false statements about his conversations with the Russian ambassador in December," Comey said ahead of his appearance before the Senate Intelligence Committee on Thursday.
The account appears to confirm media reports about Comey's interactions with Trump that are to be at the heart of Thursday's hearing, although the information had not previously come directly and publicly from Comey.
Trump fired Comey last month and questions have since swirled about whether he had pressured Comey on the ongoing Russian investigation - allegations that could amount to obstruction of justice. Trump has denied the allegations.
The prepared testimony also details other interactions between the president and FBI chief, including informing Trump he was not personally under investigation and the president's demand for "loyalty" from Comey.
Comey said he documented their meeting afterwards by typing up notes on a laptop in an FBI vehicle outside Trump Tower "the moment I walked out of the meeting".
Adam Schiff, the ranking member of the House Permanent Select Committee on Intelligence, said Comey's testimony was disconcerting.
"Former FBI Director Comey's written testimony confirms a host of troubling allegations concerning the president's conduct," Schiff said in a statement on Wednesday.
"The president sought to obtain a pledge of loyalty from the director of the FBI during a conversation that centred on whether the director would be able to keep his job."
Sarah Sanders, deputy White House press secretary, said aboard Air Force One on Wednesday that Trump hadn't had time yet to read Comey's released testimony.
"I'm not aware if he's had a chance to review any of the specific details of that testimony. I did find the timing of the release a little bit interesting," she told reporters.
Trump abruptly fired Comey last month; the dismissal sent shockwaves across Washington and was swiftly condemned by Democrats - as well as some in Trump's own Republican Party.
The administration has given a series of varying accounts for what led the president to sack Comey.