U.S. President Donald Trump checks time prior to a dinner at the Art and History Museum at the Parc du Cinquantenaire during the NATO Summit in Brussels, Belgium July 11, 2018.
May is trying to unify her deeply divided Conservative Party behind her Brexit plans with some of her own lawmakers openly speaking of a leadership challenge.
Safety alerts are normally issued to foreign citizens by their embassies in times of political instability, terrorism, and natural disasters; but the USA embassy in London has had to issue a "demonstration alert" on Tuesday ahead of planned protests for July 12 to 14.
The National Police Chiefs Council said rest days for officers had been cancelled and many would be working 12-hour shifts during Mr Trump's visit. "But when they make you feel unwelcome, why would I stay there?" "Now, for the benefit of all our people, let us work together to build a more prosperous future", she said.
This will be followed by a Guard of Honour giving the Royal Salute, with the USA national anthem being played, and Trump and the Queen inspecting the Guard of Honour.
Trump, who is making his first presidential visit to Britain, told the Sun newspaper he had advised May on how to conduct Brexit negotiations, "but she didn't listen to me".
- Mr Trump and his wife will spend Thursday night at Winfield House in Regent's Park, which is the US Ambassador's residence in London.
Ahead of his arrival the PM said she would use the visit to "forge a strengthened, ambitious and future-proof trade partnership" with the United States after Brexit.
"There is no stronger alliance than that of our special relationship with the US and there will be no alliance more important in the years ahead", May said in a statement.
This is when huge protests will be held across the city, with 50,000 people expected to turn up on Friday afternoon - parading to Trafalgar Square through central London.
A campaign is also underway to get "American Idiot", a 2004 song by the US punk rock band Green Day, to the top of the UK Singles Chart.
It is not simply the protesters, who are expected to dog Trump throughout his visit, including his weekend in Scotland, but his fraught relationship with political leaders accustomed to harmonious exchanges with US leaders, a tradition of unity that goes back at least to the vital World War II partnership of Franklin D. Roosevelt and Winston Churchill.
Before the Sun interview was published, Ms May invoked Winston Churchill as she addressed Mr Trump and business leaders at a black-tie dinner at Blenheim Palace, the grand 18th-century country house which was the British World War II leader's birthplace.
The Trumps trip to the United Kingdom is not an official state visit, however, tomorrow the couple will visit the Sandhurst military academy and hold talks at May's country house, Chequers.
Over a lunch of Dover sole, lamb and lemon meringue pie, the leaders will discuss Russian Federation, trade, Brexit and the Middle East.
The Queen will meet Mr Trump and first lady Melania Trump at the dais in the quadrangle of the historic royal residence on Friday.
On Saturday he will fly up to Scotland to play golf at one of his resorts and his son Eric landed in the family's "Trump Force One" private jet at Aberdeen, before flying to Helsinki to meet Vladimir Putin on Monday.