Nigel Farage said Joe Biden's election debate performance was “worse than anyone can believe”.
The Reform UK leader, who is a close ally of Donald Trump, told The Sun that the US President is “totally unfit for office”.
Mr. Biden faced Mr. Trump last night for a 90-minute showdown in the first televised debate of the 2024 White House race.
But the 81-year-old gave a stumbling performance as he appeared to freeze on stage.
At one point, Mr Biden trailed off mid-sentence as he said: “Making sure that we're able to make every single solitary person eligible for what I've been able to do with the, with the Covid, excuse me, with dealing with everything we have to do with, look…”
He added: “Look, if, we've finally beat Medicare.”
Labor leader Sir Keir Starmer this morning refused to comment on Mr. Biden's performance.
He told the BBC: “I have to say I've got enough on my hands with my own election campaign to go too far into that.
“In the end, the person who is president of the US will be determined by the American people in due course and if we are elected into government to serve, we will deal with whoever is the president.
“The relationship between the UK and the US is strong, it's historic and obviously it's above the individuals who are in office.”
Asked if he was worried about Mr. Biden, he said: “Well, look, I don't think me commenting on the American election is helpful. We've got our own election going on here.
“And in the end it's not for me, it's for the American people to decide and they will decide and we'll deal with whoever they elect in as their president.”
There have been ongoing questions about Mr. Biden's health and if he can manage another four years in office.