Mr Trump was one of the first world leaders to say that he would attend the funeral of Pope Francis, who died of a stroke on Monday. He is expected to fly into Rome late on Friday night.

He will be joined by Sir Keir Starmer, Emmanuel Macron of France, King Felipe VI of Spain and the president of Brazil, Luiz Inácio Lula da Silva.

Mr Trump mocked his predecessor, Joe Biden, for where he was seated during Queen Elizabeth II’s funeral, claiming it was a sign of America’s diminished stature on the world stage, and insisting he would have been placed much farther forward if still in office.

Mr Biden was seated on row 14, such was the protocol that governors-general of the realms that retain the monarch as their head of state, were seated first with and elected Commonwealth leaders behind them.

‘No respect’

Mr Trump posted on social media: “This is what’s happened to America in just two short years. No respect. However, a good time for our president to get to know the leaders of certain Third World countries.“If I were president, they wouldn’t have sat me back there – and our Country would be much different than it is right now.

“In real estate, like in politics and in life, location is everything.”

The funeral will be the most prominent gathering of world leaders since Mr Trump’s inauguration – and may throw up considerable diplomatic awkwardness.

“One of the interesting aspects of a ceremony such as this is that the Holy See has diplomatic relations with countries such as Russia and Iran, which may mean that world leaders who would not usually meet, would encounter one another in Rome, either the night before the funeral or at the actual ceremony,” said Ms Pepinster.

“But it can cause embarrassment, such as in 2005 when Prince Charles was seated close to Robert Mugabe.”