Walking to the stage at the front of the chamber, he shook hands and exchanged words with several representatives of the 120 countries who had signed a pledge supporting reform, and were therefore entitled to attend.

But when he saw Mr Johnson he stopped and greeted him warmly.

Mr Johnson engaged the president in animated conversation, which at one point led to Mr Trump shooting the Foreign Secretary a quizzical look.

Mr Trump then began his set speech - but couldn't resist opening with a plug of his own property: the Trump World Tower, which sits opposite the New York headquarters of the UN.

When Nikki Haley, the US ambassador to the UN, introduced Mr Trump and spoke of the UN's potential, Mr Trump said: "I actually saw great potential right across the street, to be honest with you, and it was only for the reason that the United Nations was here that that turned out to be such a successful project."