Final details are still being worked out and no official date for his testimony has been set. Comey is expected to appear before the Senate Intelligence Committee, which is investigating possible connections between the Trump campaign and Russia during last year's presidential election.
Comey has spoken privately with Special Counsel Robert Mueller III
to work out the parameters for his testimony to ensure there are no legal entanglements as a result of his public account, a source said. Comey will likely sit down with Mueller, a longtime colleague at the Justice Department, for a formal interview only after his public testimony.
When he testifies, Comey is unlikely to be willing to discuss in any detail the FBI's investigation into the charges of possible collusion between Russia and the Trump campaign -- the centerpiece of the probe, this source said. But he appears eager to discuss his tense interactions with Trump before his firing,
which have now spurred allegations that the president may have tried to obstruct the investigation.
If it happens, Comey's public testimony promises to be a dramatic chapter in the months-long controversy, and it will likely bring even more intense scrutiny to an investigation that Trump has repeatedly denounced as a "witch hunt."
The appointment of Mueller as a special counsel in the Russia investigation had raised concerns among some members of Congress that his probe could scuttle the chance for Congress and the public to hear directly from Comey. That appears less likely now that Mueller and Comey have discussed the limits of his testimony.
Since his firing last month, dramatic accounts have emerged in the New York Times, CNN, and elsewhere about the tense confrontations with Trump that Comey memorialized in memos afterward
A week after he took office in January, Trump allegedly demanded Comey's "loyalty"
if he kept him on as FBI director, and he urged Comey to drop his ongoing investigation into Michael Flynn, Trump's fired national security adviser, in a separate, one-on-one meeting.
The source said that Comey is expected to stand by those accounts in his testimony.
"The bottom line is he's going to testify," the source close to the issue said. "He's happy to testify, and he's happy to cooperate."
Officials with the Justice Department and Mueller's office declined to comment.