Former White House chief strategist Steve Bannon says the firing of FBI Director James Comey may have been the biggest mistake in "modern political history."

Bannon confirmed he opposed President Donald Trump's decision to oust Comey, calling the FBI "an institution." Bannon told CBS that institutions such as the U.S. Senate and House of Representatives can be changed "if the leadership is changed." But he also said the FBI is 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," Bannon said.

The ousted White House adviser also said that if Comey hadn't been fired, "We would not have the Mueller investigation," referring to special counsel Robert Mueller.

Bannon, whose far-right views on immigration, climate and trade helped shape Trump's presidential campaign and his first months in office, was also fired by the Republican president last month in a push to end factional fights within the White House.

Bannon returned to his former job running right-wing website Breitbart after departing from his White House, saying he would go "to war" for Trump.