In the murky world of intelligence there are many shades of grey, but in this case the denial could hardly have been more emphatic. Barack Obama's former director of national intelligence was crystal clear that the FBI had never been given a secret court order to carry out electronic surveillance at Trump Tower in New York.

"Absolutely I can deny it,” James Clapper said with a steady gaze, adding for good measure: "Something like this? Absolutely I would know."

The interviewer on NBC News spluttered, surprised that such a hugely serious allegation by the president of the United States had been so categorically shot down. Just to make sure, he asked again.