The next US attorney general, under Joe Biden, should not “pursue a criminal investigation of Donald Trump”, Comey writes, “no matter how compelling the roadmap left” by the special counsel Robert Mueller, or “how powerful the evidence strewn across his history of porn stars and financial fraud”.

“Although those cases might be righteous in a vacuum,” Comey writes, “the mission of the next attorney general must be fostering the trust of the American people.”

That opinion is likely to surprise many – perhaps including Trump himself – as Comey has emerged as a powerful critic of Trump and has been the frequent target of threats and insults from the 45th president.

Comey was fired by Trump in May 2017, an act that prompted Mueller’s appointment to investigate Russian election interference and links between the president and Moscow.

Comey’s new book, Saving Justice: Truth, Transparency and Trust, is his second and will be published on 12 January.