LONDON (REUTERS) - Prime Minister Boris Johnson said it was best if US President Donald Trump did not get involved in Britain's upcoming election when he visits London for a Nato summit next week.

"What we don't do traditionally as loving allies and friends, what we don't do traditionally, is get involved in each other's election campaigns," said Johnson, whose Conservative Party has a commanding lead in the polls ahead of the Dec 12 vote.

"The best (thing) when you have close friends and allies like the US and the UK is for neither side to get involved in each other's election."

Trump has already waded into the election, saying in October left-wing opposition leader Labour Party Jeremy Corbyn would be "so bad" for Britain and that Johnson should do a pact with Brexit Party leader Nigel Farage.

Corbyn has used Trump's praise of Johnson as one of his focal messages to attack the Conservatives in his campaign, saying they would sell off parts of the much-loved state-run National Health Service to US businesses after Brexit if they win the election.