The US president will make his way across the Irish Sea on Wednesday.
Donald Trump has begun a three-day trip to the UK with an attack on London's mayor - calling him a "stone cold loser".
He made the comments on Twitter this morning, before arriving in London.
Sadiq Khan said the US president shouldn't be honoured with a state visit, describing him as a "growing global threat" - which prompted a furious reaction.
A spokesperson for Sadiq Khan has described Donald Trump's tweet as a "childish" insult.
....Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a terrible job - only half his height. In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now!
— Donald J. Trump (@realDonaldTrump) June 3, 2019
Mr. Trump was greeted at Stansted Airport by UK Foreign Secretary, Jeremy Hunt - who admits he expects the unexpected from the American leader;
"He is a very controversial president and we've all got used to that over last two years.
There are no visits from President Donald Trump that don't have unexpected moments."
The US president touched down earlier this morning for a three-day state visit.
The trip will involve a variety of engagements, including attending an event to mark the 75th anniversary of the D-Day landings and meeting the royal family.
Mr. Trump will also meet with Theresa May on Tuesday, to discuss trade.
UK Police say there'll be heightened security during the visit, as protests are expected.
Speaking as he left for London, Mr. Trump told reporters the US is open to a closer economic relationship with the UK;
"There's a lot going on in the UK, and I'm sure it's going to work out very well for them.
As you know, they want to do trade with the United States.
I think there's an opportunity for a very big trade deal at some point in the near future."
But speaking about his UK meetings, political analyst Amanda Sloat says the president has the advantage.
"He has recognised Theresa May is in a vulnerable position following Brexit.
She is desperate to try to negotiate new trading relationships for the UK, and so Trump is going to try take advantage of that."
