Trump flies into British election campaign

US President Donald Trump flies into Britain next week, just days before its general election - and if earlier visits are any guide, fireworks are expected.

Trump is in town for a summit marking 70 years of the NATO military alliance, but his presence risks disrupting the campaign for the December 12 vote as it enters the final straight.

The US leader has been outspoken in his support for Britain's exit from the European Union, which Conservative Prime Minister Boris Johnson has promised to deliver at all costs.

But has been equally forthright about how it should be done - and his views are not always the same as London's.

Trump has also criticised opposition Labour leader Jeremy Corbyn, but this risks backfiring in a country where Trump himself is deeply unpopular.

The NATO summit on Tuesday and Wednesday includes a reception for leaders and their partners hosted by Queen Elizabeth II at Buckingham Palace.

Johnson, who is co-hosting the event, currently has no one-to-one meeting scheduled with Trump - and indicated he would not appreciate any interventions.

"When you have close friends and allies like the US and the UK, the best thing is for neither side to get involved in each other's election campaign," he told LBC radio on Friday.

After three missed deadlines, Britain is due to leave the EU on January 31, but the country remains divided and increasingly polarised about its future.

Trump condemned former prime minister Theresa May's "foolish" attempts to get a Brexit deal and suggested Britain leave without one - an outcome businesses warn could be disastrous.

He has hailed Johnson, who took over in July threatening to walk away from talks with Brussels, as "exactly what the UK has been looking for".

But when Johnson struck his own exit deal, Trump questioned if it would allow a new trade agreement with the US - something the prime minister has held up as one of the biggest prizes of Brexit.

The president also warned in a recent interview that left-wing Labour leader Corbyn would take Britain to "such bad places".

But Labour has relished the attacks from a man facing a threat of impeachment back home, who is greeted by protests every time he visits Britain.

It is expected to use Trump's trip to repeat its claim that Johnson's Brexit deal would open up Britain's state-run National Health Service to US firms.

The prime minister has denied this, and this week said if Washington tried to include the NHS in trade talks, "we'd walk away".

Trump meanwhile is also close to Nigel Farage, a key figure in the 2016 EU referendum whose upstart Brexit Party threatens to take Tory votes with a promise of a hard break with Brussels.

During a phone-in on Farage's radio show last month, Trump suggested he and Johnson form an electoral pact - something the Tories have strongly rejected.

UK-US relations recently have been strained, notably by different approaches to Iran's nuclear programme, climate change and China.

There have also been tensions over the death in August of British teenager Harry Dunn in a car crash with a vehicle driven by the wife of a US diplomat.

Anne Sacoolas claimed diplomatic immunity and subsequently returned home, but Dunn's family want her to come back to Britain to face justice.

Trump was also outraged by the leak of diplomatic cables written by British ambassador Kim Darroch earlier this year which described the Republican White House as inept.

Trump condemned the diplomat as a "pompous fool". When then premier May defended him, he condemned her "foolish" approach to Brexit. Darroch has yet to be replaced, and the US president has previously mooted Farage for the job.