Trump attacks London's Muslim mayor Sadiq Khan as he visits UK

US President Donald Trump arrived on his state visit to Britain on Monday in combative mood, following up his weekend interventions over Brexit with a broadside against London's "loser" mayor.

The president's plane had not even touched down when he tweeted that Sadiq Khan, who has been highly critical of the red-carpet welcome laid on for Trump, had done a "terrible job".

The president called the mayor a "stone cold loser" before adding: "In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit."

Queen Elizabeth II will welcome Trump and his wife Melania to Buckingham Palace later Monday, where they will be treated to a guard of honour, a private lunch and a glittering state banquet.

But beneath the pomp and ceremony, Britain is in turmoil with Prime Minister Theresa May due to step down within weeks over her handling of her country's exit from the European Union.

Where other leaders may have treaded lightly, Trump weighed in, declaring before he arrived that former foreign minister Boris Johnson would make an "excellent" choice to succeed May. 

In a round of British newspaper interviews, he also recommended her successor walk away from talks with Brussels, refuse to pay Britain's agreed divorce bill and leave the EU with no deal.

The much vaunted UK-US "special relationship" was already under strain over different approaches to Iran, China and climate change, as well as Trump's personal politics.

Labour's Khan has led opposition to the three-day visit, condemning Trump's "divisive behaviour" and saying he was "one of the most egregious examples" of a growing global threat from the far-right. 

Large protests are planned in London, while opposition politicians are also boycotting the state banquet on Monday night.

But May and Trump are expected to emphasise the wider benefits of the old alliance when they hold talks at Downing Street on Tuesday.

On Wednesday, they will join other world leaders in the English port of Portsmouth to commemorate 75 years since the D-Day landings, which changed the course of World War II.

"Our relationship has underpinned our countries' security and prosperity for many years, and will continue to do so for generations to come," May said ahead of the visit.

Foreign Secretary Jeremy Hunt added: "He's very controversial, he's a disruptor. But he's also president of our most important ally."

Large protests are planned in London against Trump's visit, while opposition politicians are also boycotting the state banquet on Monday night

No-deal Brexit

May announced her resignation last month after failing to get her Brexit deal through parliament and twice delaying Britain's EU departure. 

She will formally quit as her Conservative party's leader on Friday, but will stay on while they find someone to replace her.

Three years after the referendum vote for Brexit, Britain remains divided and anxious about its place in the world.

Trump recommended the new government be bold and make a clean break with the EU if necessary, adding that there was "tremendous potential" for Britain to trade with his country after Brexit.

This message chimes with those of many of the candidates seeking to replace May, including Johnson. However, others warn against severing ties with Britain's closest trading partner.

Trump said he might meet with Johnson and pro-Brexit populist leader Nigel Farage during his UK visit.

"They want to meet. We'll see what happen," he told reporters before he left the United States.

Strained special relationship

May was the first foreign leader welcomed to the White House after Trump's election victory in November 2016, but their relationship has not always been rosy.

They have clashed in the past over his migration policies, while Britain still backs the Iranian nuclear deal and the Paris climate accord, both of which Trump has abandoned.

Washington has also been putting pressure on Britain to exclude Chinese tech giant Huawei from its 5G network over security concerns, suggesting it might harm intelligence sharing.

Trump's first official visit to Britain last year was overshadowed by criticism of May's approach to Brexit, as well as large demonstrations.

Protesters are planning a big turnout this week and are hoping to once again fly their inflatable balloon showing Trump as a baby over the streets of London.

But the president's itinerary, which also includes a tea with heir to the throne Prince Charles, will keep him away from the public. 

He is not expected to meet Prince Harry and his American wife Meghan Markle, after saying her previous criticism of him was "nasty".

Agencies contributed to this report.