"Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!" Trump tweeted.

Trump has declared himself a big fan of Johnson.

Last week he predicted Johnson would fix what Trump called the "disaster" that outgoing British premier Theresa May had triggered in trying to lead Britain out of the European Union.

"He's a different kind of a guy, but they say I'm a different kind of a guy too. We get along well. I think we'll have a very good relationship," Trump told reporters.