James Comey delivered a personal assessment of Donald Trump's character in an interview with BBC Newsnight.
He said the President has a "corrosive" and "staining" effect on those around him.
Comey concluded: "I don't dislike him, I actually feel sorry for him as a person."
It is latest of many verbal blows the US President and former FBI Director have launched at each other.
James Comey has said that he doesn't dislike Donald Trump, but pities him instead.
"I don't dislike him, I actually feel sorry for him as a person," Comey said of his ex-boss in an interview with the BBC which aired on Thursday:
Comey went on to say that Trump had a "corrosive" and "staining" effect on his employees, and the nation as a whole.
He told the flagship Newsnight program:
"I think the way in which he acts, especially his corrosive effect on norms — truth-telling being the most important of them — has that staining effect on institutions and people who are close to them.
"He has a habit of — and even who support him would agree with this, I think — of telling lies, sometimes big, sometimes casual, and insisting that the people around him repeat them and believe him.
"And that stains any human."
Comey's jab at Trump on Newsnight is the latest of many verbal blows the U.S. president and ex-FBI director have exchanged since Trump fired Comey last May.