British Prime Minister Boris Johnson conceded that a no-deal Brexit remained a possibility on Friday, while also urging U.S. President Donald Trump not to get involved in the UK election.

Johnson, the 55-year-old leader of the Conservative Party, told a radio phone-in on Friday that it would be "best" if Trump did not get involved in the election when he visits London for a NATO summit next week.

Trump, a deeply unpopular figure in the UK, caused uproar in October by saying Labour Party leader Jeremy Corbyn would be "so bad" for Britain and that Johnson should strike a pact with Brexit Party leader Nigel Farage.

On Friday, a senior Trump administration official told reporters that Trump is "absolutely cognizant of not, again, wading into other country's elections."

Why are Conservatives worried about Trump?

Senior Conservatives are nervous that Trump could upset the campaign when in London, just over a week before the December 12 election, which polls indicate Johnson is on course to win.

"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," Johnson told LBC radio.

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

The Labour Party is likely to use the arrival of Trump to attempt to reinvigorate its election campaign.

Corbyn has accused Johnson of planning to sell off parts of Britain's health service to U.S. businesses after Brexit. Johnson has denied the claim. 

Trump has previously said everything including health should be on the table in trade talks, though he later said health would not be.

Is a no-deal possible?

Johnson also accepted on Friday that it was still possible the UK could leave the European Union without an agreement, saying that Britain would remain "in a state of readiness" for such a scenario.

It is the first time he has conceded a no-deal could still happen, a charge repeatedly made by opponents, during the election campaign.

A no-deal exit could take place if Johnson is unable to win a majority for his deal, or when a planned transition period – which he insists he will not extend – concludes at the end of 2020.

On Friday the prime minister said he was confident that Britain would get a free trade deal with the EU by the end of 2020, but wanted to keep in place preparations the government has made for the possibility of a no-deal Brexit.

"Many of those preparations will be extremely valuable as we come out of EU arrangements anyway," he told reporters. "So I think that they were a right thing to have done and to keep in a state of readiness."

(With input from Reuters)