"If they do that, then their trade deal with the USA will probably not be made". The U.S. can not negotiate a trade deal with the United Kingdom until that date Theresa May cited, which is in 2019.
"The relationship between our two nations is indispensable to the cause of justice and peace", Trump said.
The plan has infuriated fervent Brexit supporters, who think it would limit Britain's ability to strike new trade deals around the world.
Mr Trump arrived at the prime minister's Buckinghamshire residence by helicopter after visiting the Royal Military Academy at Sandhurst, Berkshire, on Friday morning.
On Boris Johnson, Trump said again he thought the former foreign secretary would make a great prime minister.
He added: "I would have done it much differently. I actually told Theresa May how to do it but she didn't agree, she didn't listen to me", he said. "Standing beside her today, he told a very different story".
"If the strategy in criticizing Theresa May on her own soil to her home press was to get a better trade deal with the United Kingdom, you have to just look at the facts here". And that is fine.
He also told the tabloid that he'd shared advice with May during Britain's negotiations with the European Union and she ignored it. Tens of thousands of people gathered in London for a series of demonstrations against his visit.
During a joint news conference with Ms May, Mr Trump said he had "a lot of respect for the Prime Minister".
He also referenced immigration into Europe, saying: "I think what has happened to Europe is a shame".
"I was very saddened to see he was leaving government and I hope he goes back in at some point".
"I think it betrays an ignorance that is truly alarming in a candidate that has got that far to be President of the United States".
"Whatever you do is OK with us, just make sure we can trade together, that's all that matters". (Picture: AP) The president and first lady were invited to the black-tie gala near Oxford where they're being joined by the British Prime Minister, and an array of other guests.
He added that he supported the resignations of Mr Davis and Mr Johnson, and said the Tories were telling businesses that "the rules that have burdened you for year are not going to go anywhere".