The president renewed his feud with Khan with a series of heavyhanded comments aimed at Johnson's successor as London mayor, trying to blame him for the spate of fatal terror attacks in the capital previous year at Westminster, London Bridge and outside Finsbury Park mosque. Over 50,000 people are expected to descend on central London on Friday to protest the leader's visit.
Are we even surprised the controversial president did a faux pas?
Mr Trump has previously spoken of his admiration for the Queen, telling The Sun that she is a "tremendous woman". "I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London", Trump said.
He added: "I used to love London as a city -I haven't been there in a long time". "I would have done it much differently, ' Trump said". "Take a look at the terrorism that is taking place", Trump said. "Look around. You go through certain areas that didn't exist ten or 15 years ago".
Mr Khan also defended allowing a giant balloon depicting Mr Trump as an angry baby to fly near Parliament, saying: "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".
Speaking ahead of Mr Trump's visit, Mr Khan said: "Around the world, the far-right and nationalist populists are on the rise".
Organisers of the crowd-sourced inflatable say that while Trump won't see the blimp in person, as his visit does not take in the historic Palace of Westminster, it's hoped it would come to the attention of the Twitter-loving president online. Add Donald Trump as an interest to stay up to date on the latest Donald Trump news, video, and analysis from ABC News.
"Once the Brexit process is concluded and perhaps the United Kingdom has left the European Union, I don't know what they're going to do but whatever you do is OK with me, that's your decision", Trump said, according to a live feed of his comments.