Joe Biden got confused over Rishi Sunak’s title when the pair met in the White House, welcoming him as “Mr President”.
The US President swiftly corrected himself, joking that he had “promoted” Mr Sunak.
The 80-year-old President, seated across from Mr Sunak for the bilateral meeting, said: “Well, Mr President – Mr President, I just promoted you. Mr Prime Minister, it’s great to have you back.”
Mr Sunak, who appeared to laugh off the error, has not been to the White House before as prime minister, only taking office last October.
Mr Biden told him: “In the past few months we have met each other in San Diego and then we met in Belfast and we met in Hiroshima.
“And now we’re here we’re going solve all the problems of the world in the next 20 minutes.”
This is not the first time the US President has appeared to fumble his words.
In April, during his visit to the Republic of Ireland, Mr Biden appeared to confuse the name of the New Zealand All Blacks rugby team with the Black and Tans – a controversial War of Independence-era police force in Ireland.
And while he heaped praise on the strength of the UK-US relationship on Thursday, he has not always been so diplomatic.
Last month, he claimed that he visited the island of Ireland to ensure the “Brits didn’t screw around” amid ongoing concern over the peace process and the impact of Brexit.