US President Donald Trump has congratulated Boris Johnson on becoming the leader of the Conservative Party and the UK's next prime minister.

"Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!" Trump tweeted after Johnson won a landslide victory in a members' ballot against rival Jeremy Hunt.

Trump has often praised Johnson as a friend, and touted him as a potential prime minister even while Theresa May was in office. The two men also indicated a mutual willingness to work with each other should Johnson win the leadership contest.

Speaking at an event of teenage fans in Washington, Trump continued his praise of Johnson. 

"We have a really good man is going to be the Prime Minister of the UK now, Boris Johnson. Good man. He's tough and he's smart. They're saying 'Britain Trump' they call him 'Britain Trump' and so people are saying that's a good thing. They like me over there. That's what they wanted. That's what they need."

Johnson will take over from May as prime minister after she steps down on Wednesday.

In an interview with CNN on Tuesday, Boris' father Stanley Johnson said he thinks his son will get along with Trump -- but warned that the relationship cannot be subservient.

"They have the same hairstyle," he said. "I think they will get on."

He added: "We're going have to be careful not to be too slavishly geared toward America," and insisted that "building bridges" with Europe after Brexit will also be crucial.

Trump featured heavily in the debates during the Tory leadership campaign, with Johnson's comments on Trump coming under scrutiny.

During a televised debate against rival Jeremy Hunt, Johnson refused to confirm that he would keep US ambassador Kim Darroch in his post, after Darroch's cables describing the Trump administration as "inept" and "incompetent" were leaked. Trump attacked Darroch publicly, and Darroch resigned the day after the debate.

Later in the campaign, Johnson was pressed on another Trump-related outcry -- the President's tweets suggesting that four US Congresswomen of color should "go back" to the countries they came from, even though all four are American.

The Conservative candidate received another round of criticism when he refused to say whether he thought Trump's tweets were racist. He did, however, say the tweets were "totally unacceptable," adding that he "can't understand" how an American leader would use such language.

Johnson has stressed the need to work with Trump and has gained support from the President for his tough Brexit stance. Both men have criticized Theresa May's approach, with Johnson calling for the prospect of a no-deal Brexit and Trump claiming, according to May, that he would have "sued" the EU.