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.
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 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.
By James Landale, BBC diplomatic correspondent
Since Donald Trump took office, Theresa May has had to disagree with him publicly over his decision to impose trade tariffs on EU steel, abandon the Iran nuclear deal, move the US embassy to Jerusalem, order a Muslim travel ban and retweet anti-Muslim messages from a British far right group.
In turn, the US president has described Britain as being in political “turmoil”, criticised its defence spending and shown no enthusiasm for coming to visit: in the 18 months of his presidency, Mr Trump has chosen to visit 17 other countries first.
Whatever this relationship is, it can hardly be described as special.
So this trip is about ticking a diplomatic box, getting a visit out of the way before its further delay became politically embarrassing.
And as working visits go, it is on the minimal side: no Downing Street barbecue, no cabinet visit, no speech to both Houses of Parliament.
The diplomatic aim will be to get through the visit without any gaffes, without upsetting the president, and without him saying anything disobliging about Brexit or a future trade deal.
The UK and the US do have a good relationship at an institutional level, in the fields of defence, security and intelligence. The tricky bit is always the politics and the personalities.
Mr Trump and his wife Melania 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 watch a joint counter-terrorism exercise by British and US special forces at a military base.
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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.
It has also complained that 300 officers working during the visit will have to sleep on camp beds in a gymnasium that has no hot water and restricted access to hot food.
Richard Cooke, chairman of the West Midlands Police Federation, said he was seeking an urgent national review of the accommodation.
Tens of thousands of people are expected to protest against the president in London on Thursday and Friday – and in Glasgow on Friday.
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.
Mr Trump’s friend and US media network boss Christopher Ruddy told BBC Radio 4 Today that he expects the US president will be “shocked” by the size of the protests.
Shaista Aziz, a Labour councillor in Oxford, and one of the organisers of the Together Against Trump protest in London, said the demonstration was about saying “very clearly that we reject the policies of this administration”.