He said that May's new, softer approach to Brexit - which would leave Britain sticking to a "common rulebook" with Brussels on goods - would kill off the chances of a much sought-after U.S. -U.K. trade deal.
Fresh from a North Atlantic Treaty Organisation summit where Trump chided Germany and other European nations for failing to contribute enough to defense spending, British Prime Minister Theresa May is hoping his trip will boost the close ties between their two nations and help forge a future free trade deal.
Ahead of the USA leader's visit, Khan gave permission for a 20-foot-tall "Trump Baby" blimp to be flown above Parliament Square which Trump said made him feel "unwelcome" in London. It would see Britain follow European Union rules on goods to protect cross-border trade and avoid checks on the Irish frontier, while enabling it to control migration and sign new non-EU trade deals, including in services.
May batted away questions about Boris Johnson, her former foreign minister who resigned last week after she announced her plan for a "soft" Brexit.
The President's remarks to The Sun materially damaged her biggest project on the eve of his visit, and he had gone out of his way to praise her political rival who had quit in protest at her Brexit plans earlier in the week.
Brexiteers have craved a US-UK trade deal since Trump made it clear he would seek to thrash out terms once Britain leaves the European Union on Friday March, 2019.
He added: "I would have done it much differently". If signing up to the Trump world view is the price of a deal, it's not worth paying", she tweeted, adding that Trump was "determined to insult" the prime minister."Our prime minister is so weak she still rolls out the red carpet for a man who does nothing but insult her.
"I was very saddened to see he was leaving government and I hope he goes back in at some point".
The President and his wife will travel to Windsor on Friday afternoon to meet the Queen, before flying to Scotland to spend the weekend at Trump's Turnberry golf resort which will be private.
"I guess when they put out blimps to make me feel unwelcome, (there is) no reason for me to go to London", he told the Sun newspaper.
The British government on Thursday published its long-awaited Brexit blueprint aimed at restarting stalled talks with the European Union, only for it to be panned by eurosceptics, the City of London and the USA president. I haven't been there in a long time.
"They all showed up on the 9th hole, overlooking the ocean, and I said what's going on, and all they wanted to talk about was Brexit".
President Trump also told The SunLondon Mayor Khan has "done a very bad job on terrorism" by allowing so many migrants to come to the city. Look at what is going on in London.
During the launch of the blimp, Sky Newsinterviewed Mayor Khan questioning whether the blimp was an appropriate form of protest.
Khan in the past has publicly feuded with Trump after the president criticized him over the city's response to a terror attack.
"She's a total professional", Trump said.
The president repeated his belief that immigration in Europe had caused terrorism and was eroding culture.