Former FBI deputy director Andrew McCabe has been fired two days before his scheduled retirement.
US attorney general Jeff Sessions sacked Mr McCabe on the recommendation of FBI disciplinary officials.
It comes ahead of an inspector general report which is expected to conclude that Mr McCabe was not forthcoming with the watchdog office as it reviewed the bureau's handling of the Hillary Clinton email investigation.
:: FBI deputy director Andrew McCabe quits amid Trump criticism
"Based on the report of the Inspector General, the findings of the FBI Office of Professional Responsibility, and the recommendation of the Department's senior career official, I have terminated the employment of Andrew McCabe effective immediately," Mr Sessions said in a statement.
He said investigators "concluded that Mr McCabe had made an unauthorised disclosure to the news media and lacked candour – including under oath – on multiple occasions."
Mr McCabe immediately disputed the findings in his own statement, saying he was being politically targeted and the firing was part of a Trump administration "war" on the FBI.
"I am being singled out and treated this way because of the role I played, the actions I took, and the events I witnessed in the aftermath of the firing of James Comey," Mr McCabe said, referring to the former FBI director who was fired by Donald Trump last year.
Donald Trump tweeted on Saturday: "Andrew McCabe FIRED, a great day for the hard working men and women of the FBI – A great day for Democracy.
Andrew McCabe FIRED, a great day for the hard working men and women of the FBI – A great day for Democracy. Sanctimonious James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the FBI!
— Donald J. Trump (@realDonaldTrump) 17 March 2018
"Sanctimonious James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the FBI!"
Mr McCabe had spent more than 20 years as a career FBI official, and played key roles in some of the bureau's most recent significant investigations.
:: Acting FBI chief McCabe contradicts White House claim over Comey's dismissal
However, Mr Trump repeatedly condemned him over the last year as emblematic of an FBI leadership he contends is biased against his administration.
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Mr McCabe had been on leave from the FBI since last January when he left the deputy director position, but his sacking comes just ahead of his planned retirement on Sunday, jeopardising his ability to receive his pension.
On Thursday, White House spokeswoman Sarah Huckabee Sanders told reporters the decision was up to the Justice Department but said "we do think that it is well documented that he has had some very troubling behaviour and by most accounts a bad actor and should have some cause for concern".
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