However, say security sources, Mr Steele became increasingly frustrated that the FBI was failing to take action on the intelligence from others as well as him. He came to believe there was a cover-up, that a cabal within the Bureau blocked a thorough inquiry into Mr Trump, focusing instead on the investigation into Hillary Clinton’s emails.

The New York office, in particular, appeared to be on a crusade against Ms Clinton. Some of its agents had a long working relationship with Rudy Giuliani, by then a member of the Trump campaign, since his days as public prosecutor and then Mayor of the city.

As the election approached, FBI director James Comey made public his bombshell letter saying that Ms Clinton would face another email investigation. Two days before that Mr Giuliani, then a part of the Trump team, talked about “a surprise or two you’re going to hear about in the next few days. We’ve got a couple of things up our sleeve that should turn things around”.