Conservative party leadership candidate Boris Johnson answers questions during a Conservative leadership hustings at ExCel Centre in London. File picture: Frank Augstein/AP

London - Former foreign secretary Boris Johnson is the strong favourite to become the next leader of Britain's ruling Conservatives as the party is scheduled to close a postal ballot of its 160 000 members on Monday.

The party is due to announce the result of the run-off between Johnson and current Foreign Secretary Jeremy Hunt on Tuesday.

The winner is expected to take over from Theresa May as British prime minister on Wednesday.

Polling of Conservative members by the political news website Conservative Home suggested that Johnson was likely to win about two-thirds of the votes.

He has vowed to leave the EU by the delayed exit date of October 31, with or without a deal.

Writing in Britain's Telegraph newspaper Monday, Johnson said that if we had the technology to fly to the Moon and back over 50 years ago, then now "we can solve the problem of frictionless trade at the Northern Irish border."

"There are few tasks so complex that humanity cannot solve if we have a real sense of mission to pull them off," he said.

Johnson was alluding to the so-called backstop provision, which he strictly rejects.

The backstop in the Brexit agreement is intended to guarantee that there will be no border controls between the British-administered territory of Northern Ireland - which would be outside the EU - and EU member the Republic of Ireland after Brexit.

If invoked in the case of there being no Brexit deal, the whole of the United Kingdom would initially remain in the customs union with the EU, while Northern Ireland would also remain bound by some of the EU's internal market rules, until London and Brussels find a better solution.

Speaking to Friday's Daily Express, Johnson said the three years since Britain voted by a slim majority to leave the EU "will seem like a bad dream," if he is allowed to form a new government.

"We'll get on with it and think much more about what we are going to do to unleash the talents and the potential of the whole country," he told the pro-Brexit tabloid.

Johnson has said a post-Brexit trade deal with the United States would be one of his immediate priorities as prime minister.

US President Donald Trump told reporters on Friday that he expected a good relationship with Johnson, who would do "a great job" as prime minister.

Trump said May had "done a very bad job with Brexit."

"I think Boris will straighten it out," he said, adding that he spoke to Johnson on Thursday.