Trump's comments come after an explosive interview the president conducted with The Sun newspaper, in which he publicly criticised the prime minister's Brexit strategy and suggested it would kill any chance of a US-UK trade deal.
Donald Trump's first official visit to Britain has coincided with him saying a US-UK trade deal may not be possible if Theresa May pushes ahead with her business-friendly plan for post-Brexit ties with the European Union.
According to The Sun, Trump said May not only ignored the advice he gave her on Brexit, but has gone "the opposite way", leading to "very unfortunate results".
There are street protests of Trump's first visit to Britain since taking office almost 18 months ago. He also said a rival politician, Boris Johnson, would make a "great prime minister".
Opponents of Donald Trump flew a six-meter blimp depicting the USA president as an orange, snarling nappy-wearing baby just outside the British parliament on Friday.
Trump and Theresa May faced the media on the lawn of Chequers, 66 km from central London where tens of thousands of people were staging a march in protest at the visit to Britain. "I think they agree with me on immigration", he told reporters in Brussels. "The people voted to break it up, so I imagine that's what they'll do". I haven't been there in a long time.
Speaking after talks at Chequers, the prime minister's country residence, Mr Trump said the US-UK relationship was "the highest level of special", while Mrs May said they had discussed plans for an "ambitious" trade agreement.
At the news conference, Mr Trump said: "The relationship between our two nations is indispensable to the cause of liberty, justice, and peace".
"Take a look at the terrorism that is taking place", Trump said. "He obviously likes me and says very good things about me", he added. That means he will not be in the city when the Angry Baby Trump Blimp flies above Parliament Square for two hours on Friday, July 13. At first, Mayor of London Sadiq Khan did not authorize its use, but he then changed his mind after more than 10,000 people signed a petition calling for him to do so.
Trump is meeting May for a working lunch before meeting the Queen for tea and a likely round at his Scottish golf course.
Addressing the 100-strong group, the prime minister said there was an "unprecedented" opportunity to do a deal that boosted jobs and growth in both countries.
It comes just days after Trump declined to say whether May should remain in post, said he had "always liked" Boris Johnson, who quit as foreign secretary over the Chequers agreement, and described the United Kingdom as being in "turmoil".
President Trump on Thursday blasted London Mayor Sadiq Khan as soft on terrorism and regretted that he's been made to feel "unwelcomed" in the British capital.