Donald Trump's advisers fear he may not be able to stop himself intervening in the UK election when he visits London for a Nato summit this week, despite a plea from British Prime Minister Boris Johnson.

The US president has been told Johnson does not want him to offer his support publicly, amid fears an endorsement could end up harming the Prime Minister's chances, and improving those of Labour leader Jeremy Corbyn.

But an insider told the Sunday Telegraph Trump may ignore the advice because he finds it hard to understand why his support would not be welcome.

President Donald Trump speaks during a meeting with Finnish President Sauli Niinisto in the Oval Office of the White House. Photo / AP
President Donald Trump speaks during a meeting with Finnish President Sauli Niinisto in the Oval Office of the White House. Photo / AP

"He's got the advice — but he doesn't have to take it," a person close to the president said.

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"He's very much a big fan of Boris Johnson ... he doesn't really completely understand why [publicly backing Johnson] is a concern."

Johnson said at the weekend "allies and friends" should not "get involved in each other's election campaigns".

Senior Conservatives are nervous praise from Trump could backfire. According to YouGov, 67 per cent hold a negative opinion of Trump.

In October, Trump waded into UK politics, saying Corbyn would be "so bad" for the UK, and advising Johnson to agree a pact with Brexit Party leader Nigel Farage.