Donald Trump should not be prosecuted once he leaves the White House no matter how much evidence has been amassed against him, the former FBI director James Comey writes in a new book.

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.”