In his first major interview since he was sacked by Trump last year, Comey told ABC News: "I don't buy this stuff about him being mentally incompetent or early stages of dementia."

"I don't think he's medically unfit to be President. I think he's morally unfit to be President.

He added: "Our President must embody respect and adhere to the values that are at the core of this country. The most important being truth. This President is not able to do that." 

Following the airing of the episode, the Republican Party rubbished Comey's claim, saying he would say anything to sell his recently launched book.

"The only thing worse than Comey's history of misconduct is his willingness to say anything to sell books," the GOP's statement, released through Republican National Committee, read.

Trump himself took to Twitter an hour before the interview footage was released to question Comey's loyalty.

"Unbelievably, James Comey states that Polls, where Crooked Hillary was leading, were a factor in the handling (stupidly) of the Clinton Email probe. In other words, he was making decisions based on the fact that he thought she was going to win, and he wanted a job. Slimeball!" the US President's tweet read.

"Slippery James Comey, a man who always ends up badly and out of whack (he is not smart!), will go down as the WORST FBI Director in history, by far!" another of his tweet directed at the former director added.

In what has been a longstanding issue in US politics, James Comey first grabbed the limelight in October 2016, when he sent letter to Congress telling them the FBI was reopening an investigation into Hillary Clinton's emails.

The subject of the letter mysteriously went public, costing Clinton the election.

Following Trump's appointment as the 45th President of the United States of America, Comey alleged that the former tried to make him take a pledge of personal loyalty, which has been dismissed by Trump on several occasions.

"The loyalty oaths, the boss as the dominant centre of everything, it's all about how do you serve the boss, what's in the boss' interests," Comey said.

In his recent rant against Comey, Trump reiterated his stance on the issue and tweeted, "I never asked Comey for Personal Loyalty. I hardly even knew this guy. Just another of his many lies. His “memos” are self serving and FAKE!"

After the FBI declared in March 2017 that they are investigating the alleged Russia link in the US election, Trump reportedly pressurised Comey to publicly announced that the President was excluded from the process.

Two months later, in May, the then FBI director was fired after he failed to cooperate with the President.

Image: Respective account on Twitter