She said her nation was determined to continue negotiations aimed at an amicable split that would allow for some ties to remain.
"The only thing I ask of Theresa is that we make sure we can trade, that we don't have any restrictions, because we want to trade with the United Kingdom and the United Kingdom wants to trade with us", Trump said.
The prime minister has warned rebels within her own party that there could be no Brexit at all if they defy her blueprint for leaving the EU. "So I want us to be able to sit down to negotiate the best deal for Britain".
Seventy-seven percent of respondents to a YouGov poll for ITV said they had an unfavorable view of Trump, with just 17 percent voicing a favorable opinion.
In his interview with Morgan, Trump said the United States and its longtime ally would "argue, we're going to fight, and we're going to end up making a deal".
Though Mr Trump later contradicted his comments by then promising a great U.S. trade deal, the president made clear his admiration for the 54-year-old Mr Johnson, who Trump said would one day make a great British prime minister.
"In terms of who ultimately holds the pen on the papers that go to Cabinet for collective decision, it has been the Cabinet Office's Europe Unit, and they have clearly been operating to a different ultimate goal to the one what we were operating to".
Speaking to the BBC on the Andrew Marr Show on Sunday morning, the Prime Minister finally revealed what the President had said: "He told me I should sue the EU".
She invited Conservative lawmakers who she hopes to persuade to back her Brexit plan, including Cheryl Gillan, John Penrose and Edward Leigh, to her Chequers country retreat shortly after Trump's entourage departed.
Trump lavished praise on British exports in the interview, describing them as "fantastic" as he waxed optimistic that the two countries could find common ground on trade.
Trump wish for warm Putin ties highlights policy disconnect
Trump wrote as part of a belly-aching trio of tweets sent from Air Force one as he traveled between Scotland and Finland Sunday. Meanwhile, in Helsinki, more than 2,000 people protested on Sunday as the city prepares to host the summit.
"I think maybe she found it too brutal", he said, standing alongside Mrs May, without revealing the details.
May said that Trump, who had been in Britain for a visit, had noted that since she's already entered negotiations, she should not walk away.
After meeting with May on Friday, however, Trump seemed to roll back his comments, declaring a deal would be "absolutely possible".
In a Facebook post on Sunday, May said she had "yet to see a workable alternative future trading arrangement" to compete with her proposals.
"David Davis was discussing with [EU chief negotiator] Michel Barnier".
"Brexit was the biggest event in democracy in the history of the British people... it should have been taken very seriously", he said.
He also criticised May for the way she handled negotiations with the European Union and said the deal she's pursuing "is not what the people voted on". I'd rather not tell you what that option is, but I think she might.
By warning that Brexit itself is in danger, May is sending a blunt message to the dozens of hardline Brexiteers in her party that if they sink her premiership then they risk squandering the victory of an European Union exit that they have dreamed about for decades.