The senior White House adviser Stephen Miller recently claimed the impeachment inquiry imperilling Donald Trump’s presidency was a product of the “deep state”, a conspiracy theory which holds that a permanent government of civil servants and security operatives exists to thwart the will of the people.

But according to Steve Bannon, Trump’s former 2016 campaign chair and White House strategist, a prime mover in the formation and propagation of the deep state conspiracy theory, it should not be taken seriously.

Bannon states his opinion in a new book, Deep State: Trump, the FBI and the Rule of Law by James B Stewart, which will be published on 8 October. The Guardian obtained a copy.

The “deep state conspiracy theory is for nut cases”, Bannon is quoted as saying, because “America isn’t Turkey or Egypt”.

There is a formidable government bureaucracy in the US, he adds, but “there’s nothing ‘deep’ about it. It’s right in your face.”

Bannon, a leading voice of the alt-right, left the White House in August 2017, after the president’s handling of the deadly far-right rally in Charlottesville, Virginia, came under fierce criticism.

In August, an unidentified member of the intelligence services filed a whistleblower’s complaint about Trump’s behaviour. It was processed through official channels, then withheld from Congress. On 25 September, after the House speaker, Nancy Pelosi, announced an impeachment inquiry, the complaint was released.

It detailed concern over Trump’s behaviour towards Ukraine, withholding military aid and asking for help in attempts to damage Joe Biden, a leading Democratic rival.

Trump has furiously demanded the identity of the whistleblower be revealed and insinuated that he or she should be punished with death.

Last weekend, as Trump surrogates joined in the furious offensive, Miller told Fox News Sunday he knew “the difference between a whistleblower and a deep state operative”.

Much of the deep state conspiracy theory’s power in US politics under Trump has been generated by the far-right Breitbart website, which Bannon ran before joining the 2016 Trump campaign and after his time in the White House.

On Thursday, Breitbart published a new essay by Virgil, the pseudonymous author of the original Deep State series. The title: Lessons of Impeachment, from Watergate to Monicagate to Bidengate.

In it, the author quoted Bannon, and added: “Yes, this is the deep state in action, and it’s out for blood.”

Bannon lost his role at Breitbart in January 2018, after the publication of Michael Wolff’s book Fire and Fury: Inside the Trump White House, in which he was quoted liberally.

Amid reports that he could yet return to Trump’s side, Bannon continues to fight over old ground. Miller was formerly an aide to Jeff Sessions, Trump’s first attorney general and a frequent target of Trump’s ire.

Among headline-grabbing shots loosed off in his conversations with Stewart, Bannon says he thought the firing of FBI director James Comey – supposedly a particularly malevolent deep state operator – was “maybe” the biggest mistake “in modern political history”.

This is not a new claim from Bannon, who told Wolff he told the president firing Comey would make the investigation of Russian election interference and links between Trump and Moscow “the biggest story in the world”.

Bannon has reportedly asserted the deep state is false before. In Wolff’s follow-up book, Siege: Trump Under Fire, Bannon describes advice he gave to a ghost writer working on Trump’s Enemies: How the Deep State is Undermining the Presidency, a book by Trump allies Corey Lewandowski and David Bossie.

According to Wolff, Bannon said: “You do realise that none of this is true.”