Donald Trump has arrived in the UK for a three-day visit, which will include a meeting of the defence alliance Nato.
The US president, who landed at Stansted airport on Monday night, is set to attend a reception for world leaders hosted by the Queen at Buckingham Palace later.
The summit marking the 70th anniversary of Nato starts in Watford on Wednesday.
Mr Trump’s last UK two visits attracted protests and more demonstrations are expected during this stay.
Scotland Yard has said road closures will be in place in central London during the Nato summit.
President Trump is due to have separate talks with German Chancellor Angela Merkel, French President Emmanuel Macron and Nato Secretary General Jens Stoltenberg.
And he will attend a working lunch with representatives from Estonia, Greece, Latvia, Poland, Romania, Lithuania, Bulgaria and the UK.
It is unclear whether there will be a one-to-one meeting between UK Prime Minister Boris Johnson and Mr Trump.
Foreign Secretary Dominic Raab – who will meet US Secretary of State Mike Pompeo later – said arrangements for such bilateral meetings were “always quite fluid”.
He told BBC Radio 4’s Today programme: “The role of this leaders’ summit – and we’re hosting it – is for the prime minister to bring all of our north American and European partners together and show, through Nato, we can be bigger than the sum of our parts.”
Mr Johnson and Mr Trump did speak on Saturday, when Mr Trump expressed his condolences after the London Bridge attack.
In a recent radio interview, Mr Johnson warned the US president against getting involved in the general election on 12 December. He has previously been criticised for voicing his opinions of British political leaders.
Mr Trump later said he was “absolutely cognisant” of the importance of not interfering in other countries’ elections.
Labour leader Jeremy Corbyn has called on Mr Johnson to demand that Mr Trump takes the NHS “off the table” in future talks on a post-Brexit trade deal. However, Mr Johnson said the claims were “nonsense” and the NHS would not be part of any such trade discussions.