LONDON (BLOOMBERG) - UK leadership front-runner Boris Johnson admitted that he did not know the full details of an international trade rule that he is proposing as a viable plan B for Brexit.

Mr Johnson was grilled in a BBC interview about a measure he argues could help Britain's economy cope if the country leaves the European Union without a deal in October.

He argued the UK could continue to trade tariff-free with the EU under Article 24 of the GATT - but both the EU and the World Trade Organisation have said it wouldn't work in the event of a chaotic Brexit.

"How would you handle paragraph 5C?" the interviewer asked.

Mr Johnson responded: "I would confide entirely in paragraph 5B."

"Do you know what's in 5C?" the interviewer asked.

Mr Johnson paused, before replying: "No."

Mr Johnson is ahead in polls of grassroots Conservatives who will choose the next leader. The party's members are overwhelmingly in favour of Brexit and regard Mr Johnson as more likely to deliver on the referendum result of three years ago than his rival, Foreign Secretary Jeremy Hunt.

But the interview on Friday (July 12) highlights Mr Johnson's weak spot - his critics say he's bad on details and has made blunders when he was foreign secretary by not being well enough prepared.

The BBC's Andrew Neil conducted twin interviews with the leadership rivals on Friday evening.

During the exchanges, Mr Johnson said it would be "insane, now", to say the government might not deliver Brexit by Oct 31, in a hint that he could be prepared to accept a delay at a later date.

Mr Johnson said he did not think it will be necessary to suspend Parliament to drive through a no-deal Brexit against the wishes of British politicians, while refusing to rule it out.

Mr Johnson denied that his failure to back British ambassador to the US Kim Darroch prompted the envoy to resign over leaked cables detailing his views of President Donald Trump. That came as British police opened an investigation into the leak.

Mr Hunt refused to guarantee the UK will leave the EU before Christmas, though he said he expected Brexit would be completed by then.

"I'm not going to give you those commitments," Mr Hunt said in the BBC interview.

"Prime ministers should only make promises they know they can deliver."

Mr Hunt told the BBC that Parliament may opt to rule out a no-deal Brexit, complicating any commitments around the fall deadline.

"If we get a deal, it will be on or around 31st October but I can't control what Parliament does and that's why I'm being honest with people," he said.