The crescendo of violence as the 3 November vote looms raises the stakes of a close and disputed election, Darroch said.

“It’s probably much closer on the ground to the battleground states, and it seems from the polls,” Darroch told the Guardian.

“Postal voting is clearly going to play a big part in this election, and it feels to me like the Trump campaign are building this up, especially if it’s close, to declare it rigged or invalid,” the former ambassador, now Lord Darroch of Kew, said.

“If Biden wins, there is a question whether the Trump base will really support or accept that as the outcome. Equally, if it looks like postal votes have been under counted or there is serious voter suppression you worry about the other side of the argument,” he added. “It feels very volatile.”