Speaking about the prime minister at a rally in Buckley, Wales, Mr Farage – who had entered the room to Kanye West’s 2010 song Power – told party supporters on Monday “you may well be related to one of his many children”.
During the election campaign, the PM has come under repeated fire for refusing to say exactly how many children he has. Mr Johnson is thought to be the father of five children, but has not confirmed the number in recent interviews.
Mr Farage, urging Leave voters not to vote for the Tories in constituencies where the Brexit Party is standing, said: “Somebody has to hold Boris Johnson to his word [on Brexit]. Without the Brexit Party we will not get the Brexit we voted for back in 2016.
“So we need to say to Conservative voters in constituencies like this: You may well like Boris. You may well find Boris highly entertaining.
“You may well be related to one of his many children, who knows?”
As the audience laughed, he added: “I shouldn’t have said that. Never mind. Never mind. Oh dear, oh dear.
“If you speak unscripted, anything can happen.”
Mr Farage, who last month announced his party won’t stand in constituencies held by the Conservatives, went on: “To make him walk the walk, you’ve got to vote for Brexit Party candidates in these constituencies.”
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Mr Farage also attacked Liberal Democrat leader Jo Swinson, saying: “I don’t want that Swinson girl anywhere near power, perfectly pleasant though she is.”
Ms Swinson later said: “Nigel Farage is going to discover that I am not always perfectly pleasant.
“Farage has stood for Parliament seven times and every single time the voters have rejected his toxic brand of chauvinistic, far-right nationalism.
“He knows he is not going to get anywhere nearer to power this election and is so scared that he hasn’t even bothered standing.
“He is lashing out at the real threat to his Tory-Brexit Party electoral pact: the girly swot leading the UK’s strongest Remain party.”