The stunning revelation is contained in Siege: Trump Under Fire, which will be published a week from now, on 4 June. It is the sequel to Fire and Fury, Wolff’s bestseller on the first year of the Trump presidency which was published in 2018.

The Guardian obtained a copy of Siege and viewed the documents concerned.

In an author’s note, Wolff states that his findings on the Mueller investigation are “based on internal documents given to me by sources close to the Office of the Special Counsel”.

But Peter Carr, a spokesman for Mueller, told the Guardian: “The documents that you’ve described do not exist.”