The FBI director says it makes him "mildly nauseous" to think he may have swayed last year's US presidential election.
On October 28th, less than two weeks before the election, James Comey wrote a letter to Congress indicating that emails linked to Mrs Clinton's private email server were being investigated by his organisation.
Only days before the election, he announced no evidence of wrongdoing had been found in the new emails.
The FBI had previously conducted an investigation into Mrs Clinton's use of a private server as secretary of state.
Last July, Mr Comey concluded that she had acted "extremely careless" but no criminal charges would be recommended.
Political opponents of Mrs Clinton - including Donald Trump - had frequently criticised her over the email issue, but it became a major talking point in the final days of the campaign after the bombshell FBI letter.
Yesterday, Mrs Clinton cited the letter as one of the reasons why she believes she lost the election.
She claimed: "If the election had been on October 27th, I'd be your president."
Speaking during a Senate panel today, Mr Comey said he thought concealing the discovery of new emails would have been "catastrophic".
He told senators: "It makes me mildly nauseous to think we might have had some impact on the election, but honestly it wouldn't change the decision.
"Everybody who disagrees with me has to come back to October 28th with me. Stare at this and tell me what you would do."
Earlier in the day, President Trump again brought up the email issue on Twitter:
FBI Director Comey was the best thing that ever happened to Hillary Clinton in that he gave her a free pass for many bad deeds! The phony...
— Donald J. Trump (@realDonaldTrump) May 3, 2017
...Trump/Russia story was an excuse used by the Democrats as justification for losing the election. Perhaps Trump just ran a great campaign?
— Donald J. Trump (@realDonaldTrump) May 3, 2017
Mr Comey has previously confirmed that an investigation is ongoing into alleged Russian interference in the US election campaign.
Again acknowledging the ongoing investigation, today he said: “We’re not going to say another peep about it until we’re done.
"I don’t know what will be said when we’re done, but that’s the way we handled the Clinton investigation, as well.”
