Donald Trump’s former chief strategist Steve Bannon has called the firing of FBI director James Comey potentially the biggest mistake in “modern political history”.

Mr Bannon, who left his post as executive chairman of far-right website Breitbart to run Mr Trump’s presidential campaign, was himself dismissed by the White House in August.

He is understood to have played a key role in some of the Trump administration’s most controversial policies, including the travel ban and the decision to pull out of the Paris Agreement on climate change.

Making his first TV appearance since being ousted from the President’s team, Mr Bannon appeared on CBS’s 60 Minutes, where he discussed the firing of Mr Comey, who had been leading the investigation into possible collusion between Mr Trump’s advisors and the Russian government during the 2016 presidential campaign.

Mr Bannon confirmed on the show that he opposed Mr Trump’s decision to remove the former FBI director. He called the FBI “an institution” and said that while bodies such as the US Senate and House of Representatives can be changed “if the leadership is changed”, the FBI was different.

“I don’t believe that the institutional logic of the FBI, and particularly in regards to an investigation, could possibly be changed by changing the head of it,” Mr Bannon said.

Presenter Charlie Rose told Mr Bannon during the interview: “Someone said to me that you described the firing of James Comey – you’re a student of history – as the biggest mistake in political history”.

Mr Bannon responded by saying: “That probably would be too bombastic even for me, but maybe modern political history.”

“If you’re saying that that’s associated with me then I’ll leave it at that,” he added.

Mr Bannon, who returned to Breitbart after being ousted from the White House, said he believed the current investigation conducted by special counsel Robert Mueller would not be taking place had Mr Comey not been fired.

“We would not have the Mueller investigation and the breadth that clearly Mr Mueller is going for,” he said.

Mr Mueller, who has taken over the investigation into Russia’s alleged meddling in the election, is said to be examining the draft of a letter written by Mr Trump that allegedly outlines his reasons for firing Mr Comey and could provide evidence of the president acting in a way that obstructed justice.

He has reportedly alerted the White House of his intention to interview former press secretary Sean Spicer, ex-chief of staff Reince Preibus and communications director Hope Hicks as part of his investigation.