"When a deal takes so long, they never work out very well". Trump attended the summit, where he provoked a crisis session to force allies to raise their defence spending.
May responded to Trump's remarks by saying: "We have come to an agreement at the proposal we're putting to the European Union which absolutely delivers on the Brexit people voted for".
The plan has infuriated fervent Brexit supporters, who think sticking close to the bloc would limit Britain's ability to strike new trade deals around the world.
"The people voted to break it up so I would imagine that's what they would do, but maybe they're taking a different route, I don't know if that is what they voted for".
The comments were published on Thursday as May hosted an elaborate state dinner for the president and his wife Melania in honour of their arrival in the country.
In a statement, Ms Sanders said: "The President likes and respects Prime Minister May very much".
"I shouldn't be the arbiter, as a politician, of what's in good taste or bad taste, what's important is it to be peaceful, and for it to be safe", he said.
More than 60,000 people have signed up to demonstrate in London on Friday when protesters intend to fly a large balloon over parliament portraying Trump as an orange, snarling baby.
Trump claims a phantom victory in North Atlantic Treaty Organisation spending war
They are also simplistic and crude - and based on Angela Merkel's cool reaction , it looks like they missed their mark. Trump's comments came in a wide-ranging news conference after attending a special meeting on defense spending.
In Scotland, almost 8,000 people had signed a petition to be allowed to fly the giant Trump baby ballon over the Turnberry course where the USA president is expected to play golf Saturday.
Trump went on: "You have a mayor who has done a awful job in London".
He added that Europe as a whole was "losing its culture" because of immigration from the Middle East and Africa. The Trumps will be staying with Johnson on Thursday night, one day before a mass demonstration against Trump's policies is set to take place in London. "They like me a lot in the UK".
Thornberry described Trump's comments as "rudeness upon rudeness upon rudeness", and urged May to stand up to him.
"Paris, Nice, Brussels, Berlin. This is not the way you behave", Thornberry told broadcaster ITV. May had used the Blenheim black tie dinner with political and business leaders to press Trump on the benefits of a free trade deal after Brexit.
In contrast to the president's sharp words, Trump's first event in England was an oasis of warm greetings at an evening reception at Blenheim Palace, birthplace of Winston Churchill, the larger-than-life British leader cited by the president as a model of leadership.
U.S. President Donald Trump has made the grave error of referring to Ireland as a part of the United Kingdom.. Police worked overtime, their days off cancelled. "We urge that the Scottish government rules out any use of Prestwick by the president or his entourage, and so send the most powerful message possible that Donald Trump is not welcome in Scotland".
The blimp is part of a series of protests across the United Kingdom organized by the Stop Trump Coalition, which describes itself as a "coalition of organizations and individuals joined forces to protest against Donald Trump's planned visit to the UK".
The mood was far less jovial in Belgium earlier in the day.
The agenda for Trump and May on Friday includes discussion on relations with Russian Federation, ahead of a summit in Finland on Monday between Trump and President Vladimir Putin.