Speaking at his daily White House coronavirus briefing, the president was asked about his decision to fire Michael Atkinson, the intelligence community inspector general who passed the whistleblower's report to Congress.
Mr Trump said that Mr Atkinson had done a "terrible job" and complained that the report was a hoax, even though it was later substantiated by a partial phone transcript released by the White House and by the testimony of a number of serving or former Trump administration officials.
The president told the briefing: ""That man is a disgrace to IGs. Let's go. Next. He's a total disgrace." He complained that Mr Atkinson had never come to see him, adding: "He's not a Trump fan, I can tell you that."
Mr Trump then moved on to the whistleblower. Pointing at reporters he said: "You know who the whistleblower is, and so do you, and so does everybody in this room, and so do I. Everybody knows.
"But they give this whistleblower a status that he doesn't deserve. He's a fake whistleblower. And frankly, somebody ought to sue his ass off."
The whistleblower, who is legally entitled to anonymity, is thought to be a CIA agent who was working at the White House when members of staff expressed concerns to him over Mr Trump's 25 July call to his Ukrainian counterpart, Volodymyr Zelensky.
In the call Mr Trump asked him to launch investigations into his probably Democratic opponent in November's election, Joe Biden, and his son Hunter Biden. He also appeared to threaten the former US ambassador to Ukraine, Marie Yovanovitch, who was fired after apparently standing in the way of a scheme by the president's personal lawyer, Rudy Giuliani, to discredit the Bidens.
Mr Trump, referring to her as "the woman" said: "Well, she's going to go through some things."
The whistleblower's report prompted a series of private and public congressional hearings that led to Mr Trump being impeached on charges of abuse of power and obstruction of Congress. However, he was acquitted on those charges by Republican allies in the Senate.
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