Trump is scheduled to leave for Scotland Friday evening. And, although he insisted he was not trying to pit Johnson against May, he added: "I am just saying I think he would be a great prime minister".
The UK administration, he said, has a "remarkable ability to play the ball and not the man so they will talk about the ways in which the (Brexit) deal the Prime Minister is proposing could allow for a free trade arrangement with the United States".
But the President was also clear to express that his favourable impression of Johnson did not mean the time for the end of May's leadership had come.
"The idea that we would park our rights and privileges and freedoms because it may cause offence to a USA president, I think people in London and the United Kingdom would find objectionable as indeed would Americans", he said. "I actually told Theresa May how to do it but she didn't agree, she didn't listen to me", he said.
May was at pains to impress Trump and planned to push for a trade deal in their meeting - an accord which had seemed like a foregone conclusion in the early stages of their relationship.
Trump was greeted by massive protests across Britain, including tens of thousands of demonstrators who filled the streets of London alongside a giant balloon that flew over Parliament on Friday depicting him as a cell-phone-toting angry baby in a diaper.More news: London pub renames itself The Trump Arms ahead of presidential visit
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London Mayor Sadiq Khan said the demonstrations are not anti-American but rather reflect opposition to "the politics of fear and despair".
After the diplomatic explosion of the overnight interview, May and Trump sought to play up their closeness, referring to each other by their first names. "I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London". The flying of the "Trump Baby" blimp was funded via a crowdfunding project with over £30,000 having been raised so far.
Mr Trump is due to meet Vladimir Putin, who has rejected the nerve agent claims, at a summit when he finishes his four-day visit to Britain.
"The president likes and respects Prime Minister May very much", she said.
Trump blamed London mayor Sadiq Khan for allowing the protests and blasted the city's first Muslim leader as weak on terror and crime.
"If you look, she probably went the opposite way".