"Donald Trump is childish, trivial, insulting".
The US is Britain's single largest trading partner and both countries have been interested to open up their markets to encourage free trade since long. After a private lunch with the monarch, he will look at some of her artworks, and then travel to Westminster Abbey, where he will lay a wreath at the Tomb of the Unknown Soldier.
Most presidents would go out of their way to avoid such sensitive topics at a moment of extreme political stress.
The experts add that they believe honouring the U.S. president with a state visit is "incompatible" with Britain's leadership on climate change.
Mr Trump said the the United States was already the UK's largest trading partner and that he believed there was scope for further expansion.
Prime Minister Theresa May, 62, was the first foreign leader to visit Trump after his own inauguration, and his visit coincides with her final days in office: She steps down Friday after proving unable to negotiate a Brexit deal that was acceptable to Parliament.
Ahead of the visit, Trump praised Johnson, saying he would make a "very good" prime minister, and advised a sharp exit from the European Union on October 31 with or without a deal.
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Johnson, who has said he would be prepared to take Britain out of the European Union without a deal on 31 October, has not commented on Trump's latest endorsement.
Trump, who endorsed Johnson over the weekend and said he would "do a very good job - he would be excellent", landed in the United Kingdom earlier today on an official state visit.
A source close to the Environment Secretary told the Press Association: "Mr Gove was asked last night by Mr Trump's team if he would be able to meet the president today".
Britain is ready to welcome US President Donald Trump.
"Big Trade Deal is possible once United Kingdom gets rid of the shackles".
The visit has triggered calls for mass protests in London, set for Tuesday.
In it, he is seen touching on issues known to be important to the 160,000 or so Conservative members who will have the final vote on Britain's next leader, once the candidate list has been whittled down by Conservative lawmakers.
Talks with the prime minister at Downing Street will follow, ahead of a joint press conference.