The 81-year-old acknowledges that he does not talk as “smoothly” as he used to, but says he remains at the top of his game in private meetings about government policy.

In an interview with ABC News on Friday, he said only the “Lord Almighty” could tell him to stand down from the campaign, stressing the importance of beating Donald Trump in November’s election.

Asked repeatedly by the network whether he would take a cognitive test to disprove his critics, Biden said he showed his mental abilities “every day” through his work and had not been asked by doctors to complete one.

“No one said I had to,” he said. “No one said… they said I’m good.”

His mental state has faced increased scrutiny in recent days, and Biden has further fuelled speculation by making further gaffes.