"I hope you can let this go," Comey wrote, quoting Trump in the document, which CNN has not viewed but which was described by the sources.
The bombshell revelation Tuesday escalated the already raging political crises engulfing the White House triggered by the bureau's probe into alleged cooperation between Trump aides and Russia and new reports that Trump divulged classified information to top Russian officials.
Taken at face value, the disclosures about the Comey memo appear to provide the clearest sign yet that Trump tried to pressure the FBI and the Justice Department over the Russia investigation.
The stunning new developments triggered immediate claims from Democrats that Trump was guilty of obstructing justice and extreme concern from Trump's Republican allies on Capitol Hill.
The sources said that Comey was so appalled by the request from Trump, at an Oval Office meeting on February 14, that he wanted to document it in order to share his recollections of the encounter with senior FBI officials.
The White House, fighting back against yet another extraordinary political imbroglio despite its own compromised credibility, rejected the new allegations, first reported by The New York Times.
One official said that a "conversation of that nature" did not happen.
"While the President has repeatedly expressed his view that General Flynn is a decent man who served and protected our country, the President has never asked Mr. Comey or anyone else to end any investigation, including any investigation involving General Flynn," a White House official said in a statement. "The President has the utmost respect for our law enforcement agencies, and all investigations. This is not a truthful or accurate portrayal of the conversation between the President and Mr. Comey."
Interactive: The many paths from Trump to Russia
But the reports marked a grave turn in the drama surrounding the Trump presidency and put a new complexion on the President's firing of Comey last week, a move he later confided to NBC News was motivated by his anger over the Russia investigation.
CNN senior legal analyst Jeffrey Toobin summed up the magnitude of the crisis
by saying: "Three words: obstruction of justice."
"Telling the FBI director to close down an investigation of your senior campaign adviser for his activities during your campaign for president, if that's true, that is obstruction of justice."
" 'Close it down' is an instruction to stop investigating President Trump's campaign. Richard Nixon was impeached in 1974 for telling the FBI to stop an investigation of his campaign. That's what Watergate was," Toobin added. "If (Comey's) telling the truth, I don't know how anyone can see this comment as anything but obstruction of justice."
A former Justice Department official said that Comey's step in recording the details of the White House meeting was "not out of character," especially if he was concerned about the legality or moral issues.
'A number of memos'
The encounter between Comey and Trump unfolded after the FBI director was in the Oval Office briefing the President along with Vice President Mike Pence and Attorney General Jeff Sessions, according to a source close to Comey who has a copy of the memo. After the briefing, Trump "asked Sessions and Pence to leave," the source told CNN.
According to Comey's memo, the President said: "I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go."
He told Comey that Flynn, who was fired for lying to Pence about the content of his telephone conversations with Russian ambassador Sergey Kislyak before the inauguration, hadn't done anything wrong.
Comey was "concerned" that the President was trying to "stop the investigation," the source told CNN. "He wrote a number of memos, a great many if not all were about contacts with Trump -- particularly the ones that made him feel uneasy."
The source did not know how many memos Comey had written. But the former FBI director hopes that the President's threats about "tapes" of their conversations delivered last week indicate there are actually recordings.
"He would love to have them," the source said. "One of his reasons for writing these memos is the concern this couldn't be corroborated -- but that could be met if there are tapes."
Wallowing in despondency
Inside 1600 Pennsylvania Avenue, the mood was darker still.
"This is on him," meaning Trump, a top Republican close to the White House said, after speaking with dejected advisers inside the West Wing.
Even before news of the Comey memos broke, the West Wing was wallowing in despondency, amid talk of staff shakeups and as shock still percolated following reports that Trump divulged top secret intelligence to two top Russian visitors to the Oval Office last week -- a development that focused unflattering attention on the President's basic competence.
Reports seeped out of the White House that Trump is incandescent with the performance of his staff and is mulling a huge overhaul of the top officials who surround him.
Another day of crisis at the White House further distracted top officials as they prepare Trump for his crucial first overseas trip, which is now all but certain to be overshadowed by the Comey memos, but begins on Friday when the President leaves for Saudi Arabia.
Top staff, including Pence, left the White House to attend a fundraiser for the America First Policies PAC at a Washington steak house, Del Frisco's.
White House adviser Kellyanne Conway, chief of staff Reince Priebus, Trump's director of legislative affairs Marc Short and Energy Secretary Rick Perry are also in attendance, a source told CNN.
But it wasn't completely lonely: A source close to the White House confirmed former Trump campaign manager Corey Lewandowski spent time at the White House Tuesday night.
Congress in shock
Tuesday's developments sent shockwaves up to Capitol Hill, where there has been increasing disquiet among Republicans about the catalogue of missteps and self-inflicted wounds inflicted so far during the Trump presidency, which seems to be falling further and further into disarray by the day.
The most crucial question now centers on whether Republicans begin to tire of the constant dramas emanating from the White House and edge away from a President who appears politically compromised less than four months into his term.
In an ominous sign for Trump, Rep. Jason Chaffetz, chairman of the House Oversight and Government Reform Committee, sent a letter from the FBI
asking for the handover of "all memoranda, notes, summaries and recordings referring or relating to any communications between Comey and the President."
House Speaker Paul Ryan has endured a rocky relationship with Trump but has stood by the President during his trials so far in the administration. But there was no attempt to rush to the President's defense as he digested the latest stunning news.
"We need to have all the facts, and it is appropriate for the House Oversight Committee to request this memo."said Ashlee Strong, a Ryan spokeswoman.
The mood among Republicans was dark.
GOP Rep. Kevin Cramer, a Trump supporter, complained that the White House keeps changing its story on big controversies and lashed out at the administration ford a lack of "message discipline."
Another House Republican said that there was a near-universal sense of concern about each latest report: "I talked to many who are just fed up and want to move on."
Another senior House GOP reacted to the latest debacle from the White House by asking: "What the heck is happening over there?" adding that it was time for Republicans to distance themselves politically from Trump. But on the question of impeachment, this aide said the House GOP isn't there yet.
"Gross incompetence is not an impeachable offense," the aide said, adding impeachment would be a "big lift."
Sen. Lindsey Graham, a South Carolina Republican, said that the former FBI director should testify before Congress about his conversation with Trump.
House Republican Conference Chairwoman Cathy McMorris Rodgers said that the White House needs to answer questions about what happened and that Comey needs to testify "as soon as possible."
Senate Democratic Minority leader Chuck Schumer meanwhile delivered an ominous warning as he sought to pile pressure on Republicans who have been serving as a shield for Trump.
"The country is being tested in unprecedented ways. I say to all of my colleagues in the Senate, history is watching," he said.