The Reform UK leader stood and raised his right hand while taking an oath.
It came moments after Jamie Raskin, the Democrat representative for Maryland’s 8th district, laid into Nigel Farage.
Mr Farage was branded a “Putin-loving free speech impostor and Trump sycophant” by the US congressman.
Mr Raskin echoed Sir Keir Starmer’s criticism of the Reform UK leader for missing Prime Minister’s Questions in the Commons to fly to America to voice concerns about free speech in Britain to politicians in Capitol Hill.
Mr Raskin said: “He should go and advance the positions he’s taking here in Congress today, in Parliament, which is meeting today, if he’s serious about it.
“To the people of the UK who think this Putin-loving free speech impostor and Trump sycophant will protect freedom in this country, come on over to America and see what Trump and Maga are doing to destroy our freedom… you might… think twice before you let Mr Farage make Britain great again.”
Earlier at PMQs, Sir Keir had said, in response to a question about Reform UK’s opposition to the Online Safety Act, that the “honourable member for Clacton is not here representing his constituency, in the House he was elected to.”
He said Mr Farage had instead “flown to America to badmouth and talk down our country” and suggest the Americans impose sanctions on the UK, adding that “you cannot get more unpatriotic than that, it’s a disgrace”.
