US President Donald Trump leaves on Monday for a NATO summit in London and he is under pressure from Prime Minister Boris Johnson to resist the temptation to wade into the British election campaign.

As a presidential candidate in 2016 and then as president since early 2017, Trump has shown no restraint in showing support for Britain’s exit from the European Union and critiquing the politicians involved in the country’s long-running Brexit debate.

But with Mr Johnson leading polls as he faces December 12 elections, the prime minister who is hosting the London NATO summit wants Mr Trump to mind the guardrails, putting Mr Trump in the unusual position of trying to avoid his usual impulse to comment on whatever he wishes.

Mr Trump waded into the election in October by saying opposition Labour Party leader Jeremy Corbyn would be “so bad” for Britain and that Mr Johnson should agree on a pact with Brexit Party leader Nigel Farage.

Mr Johnson’s pressure prompted the White House to stress, as a senior administration official said, that Mr Trump “is absolutely cognisant of not, again, wading into other country’s elections”.

The NATO summit takes place as Mr Trump battles an effort led by Democrats who control the US House of Representatives to force his removal from office through impeachment over his pressure on Ukraine to investigate Democratic rival Joe Biden.

Mr Trump, who got back to the United States on Friday from a whirlwind trip to Afghanistan, arrives in London on Monday night (local time) for two days of meetings with NATO leaders gathered for the summit.

He will have separate talks with German Chancellor Angela Merkel, French President Emmanuel Macron and NATO Secretary General Jens Stoltenberg and attend a working lunch with representatives from Estonia, Greece, Latvia, Poland, Romania, Lithuania, Bulgaria, and the United Kingdom.

US officials see the NATO summit as a celebratory moment for Mr Trump as his pressure on member nations has led many to increase their military spending.

He is expected to seek support from member nations to increase pressure on China for what the United States sees as Beijing’s expansionist policies.

-AAP