United States President Donald Trump was due to arrive in the United Kingdom on Tuesday for an unprecedented second state visit, with police saying they have prepared for "every eventuality" in their security operation.

The president and first lady Melania Trump are being hosted by King Charles III and Queen Camilla at Windsor Castle, the historic royal residence, about 40 kilometers west of London.

On Wednesday, the president will be treated to the customary display of full royal pageantry, including a carriage procession and state banquet. On Thursday, UK Prime Minister Keir Starmer will host Trump at Chequers, his countryside retreat in Buckinghamshire, northwest of London.

Police in the UK say they are "content" with their security plans and are prepared to respond to a potential "very high threat level".

"This is a significant policing and security operation, as would be expected for a state visit of the president of the United States," Assistant Chief Constable Christian Bunt of the Thames Valley Police told reporters on Monday.

"We have planned a very comprehensive policing and security operation that has taken into consideration just about every eventuality," he added.

Having originally planned to stay in the UK until Friday, Trump's trip has since been shortened, due to the president's schedule, and his visit will now end on Thursday.

It is the first time Trump has met with Charles since he became king, but a second state visit is unusual as second-term US presidents are typically offered only tea or lunch with the monarch. Trump made his first full state visit to the UK in 2019, when he met Queen Elizabeth II.

This UK visit comes two months after Trump made a private trip to Scotland, where he met Starmer, European Commission President Ursula von der Leyen, and Scotland's First Minister John Swinney, and opened a new luxury golf course. Locals had complained about heavy policing that cost taxpayers' money during his trip to Scotland.

Key issues

Trade, Ukraine and Gaza are likely to top Thursday's agenda for talks when Trump meets Starmer. They may also discuss the recent dismissal of Peter Mandelson, the former British ambassador to Washington, over his past ties to convicted sex offender Jeffrey Epstein.

A spokesperson for Starmer told reporters on Monday that the prime minister "sees this state visit squarely through the lens of delivering for working people", citing US investment pledged over the weekend to create 1,800 jobs in Britain and a civil nuclear partnership that could lower energy prices.

Senior US officials said on Monday that US and UK plan to unveil more than $10 billion in economic deals during the visit.

The package is expected to include a three-pillar trade arrangement: a new partnership to bolster both countries' tech sectors, expanded cooperation on civil nuclear power, and deeper collaboration on defense technologies, according to a briefing with reporters.

On Tuesday, Google announced plans to invest $6.8 billion in the UK over the next two years for infrastructure and scientific research, including a new data center near London.

Also waiting for Trump in London is opposition.

A network of more than 50 trade unions, charities, and advocacy groups plans to assemble on Wednesday in central London, before marching to Parliament Square.