The "dossier", a 35-page document containing raw intelligence complied by former British intelligence agent Christopher Steele, includes a detailed and graphic account of the alleged encounter and even raises the possibility that it could have been captured on video.
A combination of file photos show U.S. President Donald Trump in the White House in Washington, DC, U.S. April 9, 2018 and former FBI Director James Comey on Capitol Hill in Washington, U.S., June 8, 2017. Mr Trump has said there was no collusion.
As of his firing, Comey said, the information remained "unverified".
When Comey first brought up the dossier after an intelligence meeting, Trump reportedly interrupted Comey, "asking - rhetorically, I assumed - whether he seemed like a guy who needed the service of prostitutes", Comey writes.
Trump has relentlessly bashed Comey on Twitter, saying Friday that he was an "untruthful slime ball" and that "Comey should be prosecuted" for leaking classified information.
The dossier and its allegations have caused great consternation for the president since their existence was revealed last January and Trump has tweeted extensively about it.
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Comey's comments come in an interview excerpt, below, released by ABC News Saturday.
Demand for Comey's stories about his interactions with Trump has been at a high since Comey was sacked by the president last May.
Trump used the investigation as a cudgel in the campaign and repeatedly said Clinton should be jailed for using a personal email system while serving as secretary of state.
"We've assembled a top team of lawyers who are going to look at that and debate it, and that's why we're putting James Comey on trial this Sunday".
Mr Trump fired Mr Comey in May 2017, setting off a scramble at the Justice Department that led to the appointment of Robert Mueller as special counsel overseeing the Russian Federation investigation.
More recently, Trump has complained that the Federal Bureau of Investigation treated Clinton more leniently than it has treated his associates during the probe into Russian interference in the presidential election. The loyalty oaths. The us-versus-them worldview. Really weird. It was nearly an out-of-body experience for me.
Comey writes that Trump offered several explanations at different times, telling Comey at one point that the story couldn't be true because, "I'm a germaphobe".