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 next week, despite a plea from Boris Johnson.

The US president has been informed that Mr 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 Jeremy Corbyn.

But an insider told the Telegraph that Mr Trump may simply ignore the advice on the spur of the moment because he finds it hard to understand why his support would not be welcome.

"He’s got the advice but he doesn’t have to take it, he gets to do his own thinking," a person close to the president said. "He’s very much a big fan of Boris Johnson, and the president won’t think he’s harming anyone else by telling everyone what he thinks. He doesn’t see that as an issue.

"He wants to help Boris Johnson not hurt him, but he doesn’t really completely understand why this [publicly backing Mr Johnson] is a concern."

As to the protocol of not giving an opinion on allies' elections, Mr Trump was said to "not think he’s breaking any rules" by doing so.

Mr Johnson diplomatically said this week that "allies and friends" should not "get involved in each other’s election campaigns."

Senior Conservatives are said to be nervous that overly fulsome praise from Mr Trump could backfire.

According to YouGov only 18 per cent of people in the UK have a positive opinion of the US president, with 67 per cent holding a negative opinion.

Mr Corbyn is also expected to use Mr Trump's presence ahead of the Dec 12 election to make claims about the NHS being "up for sale" in a US-UK trade deal after Brexit.