Sarah Sanders, the former press secretary to Donald Trump, has apologised after mocking Joe Biden’​s reference to stuttering during Thursday night’s Democratic presidential debate.

Mr Biden, who has carried a speech impediment throughout his life, was discussing those individuals who have reached out and asked him for help over the course of his political career.

“My wife and I have a call list of somewhere between 20 and 100 people,” he said towards the end of the debate. “I give them my personal phone number.” The former vice-president then described how a “little kid” with a stutter had once asked him: “I, I, I, I, I can’t talk. What do I do?’”

Failing to spot Mr Biden’s reference to his own personal experiences, Ms Sanders tweeted: “I I I I I I I I I I I I I I I hhhave (sic) absolutely no idea what Biden is talking about. #DemDebate”

Mr Biden, who last month opened up on his speech impediment in an interview with the Atlantic, replied on Twitter: “I’ve worked my whole life to overcome a stutter. And it’s my great honor to mentor kids who have experienced the same. It’s called empathy. Look it up.”

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Ms Sanders attracted fierce criticism for her comments before insisting she was “not trying to make fun of anyone with a speech impediment. Simply pointing out I can’t follow much of anything Biden is talking about.”

The ex-White House official, who left her role as press secretary in July, later deleted her original tweet and issued a second apology: “I actually didn’t know that about you and that is commendable. I apologize and should have made my point respectfully.”

During a Democratic presidential debate in July in Detroit, questions were raised of Mr Biden’s health after he appeared to stutter during a discussion on health care.

The 77-year-old addressed the moment in his interview with the Atlantic while admitting his boyhood stutter used to be a source of “shame” for him. He also revealed how he had been left with “anger, rage, humiliation” after once being mocked by a schoolteacher.