US President Donald Trump will meet Theresa May later as she pushes for a post-Brexit trade deal just days after he said the UK was in “turmoil”.
Mr Trump, who arrives in the UK this afternoon for a two-day working visit – his first since his election in 2016 – will also spend time with the Queen.
Extra security has been put in place as large-scale protests are expected.
His visit comes as Mrs May publishes a White Paper setting out a blueprint for the UK’s relations with the EU.
Earlier this week Mr Trump said it was “up to the people” whether the PM stayed on after two cabinet ministers resigned within hours over her Brexit policy.
He added he had “always liked” Boris Johnson, who quit as foreign secretary.
BBC diplomatic correspondent James Robbins described the trip as “the most controversial visit ever made by an American president to Britain”.
Robbins said it was also “really noticeable that he will barely spend any time in London” – perhaps to avoid the protests.
Mrs May said the visit would be an opportunity to boost trade links and strengthen co-operation on security.
But she also warned Mr Trump not to ignore the “malign behaviour” of Russia when he meets Vladimir Putin in Helsinki next week.
Mr Trump had joked earlier this week that his meeting with the Russian leader “may be the easiest” part of his European trip.
Along with trade and security links, Downing Street said the other key areas to be discussed between the two leaders included Brexit and the Middle East.
Ahead of Mr Trump’s visit, Mrs May said: “As two nations – we are safer, more prosperous and more creative when we work together and I am looking forward to this week’s important discussions.”
She said that when the UK leaves the European Union “there will be no alliance more important in the years ahead”.
Downing Street insisted Mrs May welcomed Mr Trump’s decision to “engage” with the Russian leader in the Finnish capital on Monday.
However No 10 also made clear that she expected him to raise issues like the Salisbury nerve agent attack on former spy Sergei Skripal and his daughter, Yulia.
Mrs May said: “Our long term objective remains a constructive relationship with Moscow. If we are to be successful in this we must keep engaging individually and as an alliance.
“That is why I welcome President Trump’s forthcoming meeting with President Putin – open channels of communication between the US and Russia are key to managing the risks of confrontation.”
Mr Trump will arrive in the UK with his wife Melania on Thursday afternoon, following the Nato summit.
The couple will attend a dinner, hosted by Mrs May, at Blenheim Palace – the ancestral home of Sir Winston Churchill – on Thursday evening.
Cabinet members, including Chancellor Philip Hammond and the new foreign secretary, Jeremy Hunt, as well as business leaders, will be among the guests.
On Friday, Mrs May and Mr Trump will go to to watch a joint counter-terrorism exercise by British and US special forces at a military base.
The pair will then travel to Chequers – the PM’s country residence in Buckinghamshire – for talks with the foreign secretary.
The president and first lady will travel to Windsor on Friday afternoon to meet the Queen, before flying to Scotland to spend the weekend at Mr Trump’s Turnberry golf resort. This part of the visit is being considered private.
The Police Federation has warned the visit will put “unquestionable pressure” on UK police forces.
Tens of thousands of people are expected to protest against the president in London on Thursday and Friday – and in Glasgow on Saturday.
Police forces from across the country have been asked to send officers to assist the Metropolitan Police.
Smaller demonstrations are also expected to be held across the UK, including Devon, Dundee, Edinburgh, Belfast, Norwich, Manchester, Leeds and Liverpool.
Meanwhile, London Mayor Sadiq Khan has granted permission for a giant inflatable figure depicting Mr Trump as a baby to fly over Westminster for two hours on the second day of the president’s visit.