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Boris Johnson has admitted he doesn't know what is in his own Brexit plan in a car crash TV interview.

The Tory leadership favourite came unstuck over his grand plan to leave the EU in a brutal grilling by the BBC's Andrew Neil.

Mr Johnson wants to use a rule known as 'GATT 24' to maintain trade with the EU if we leave without agreement on October 31.

Showing off to Mr Neil, he claimed he'd secure a "standstill" trade arrangement using Paragraph 5(b) of Article 24 of the General Agreement on Tariffs and Trade.

But Paragraph 5(c) - which comes straight afterwards - makes it clear the policy can't be used for a No Deal Brexit .

That's because it says a "plan and schedule" must already be in place to form a customs union or free trade area, "within a reasonable length of time."

(Image: AFP/Getty Images) (Image: AFP/Getty Images)

Boastful Mr Johnson tonight insisted he could pursue the plan, despite experts and the Governor of the Bank of England saying it is impossible.

He mocked Mr Neil, who mis-stated it, saying: "Paragraph 5B. Article 24. Get the detail right. Get the detail right Andrew!"

But his teasing backfired as Mr Neil immediately replied: "And how would you handle Paragraph 5C?"

Mr Johnson replied: "I would confide entirely in paragraph 5B which is enough for our purposes."

Asked directly if he even knows what's in 5C, Boris Johnson - who is set to become Prime Minister in 12 days - replied: "No."

Mr Neil scoffed: "I thought you were a man of detail!"

(Image: AFP/Getty Images)

The General Agreement on Tariffs and Trade (Gatt) is a post-Second World War legal agreement that forms the basis for the trade in goods around the world today.

It was replaced by the World Trade Organisation (WTO) in the 1990s, although the original Gatt text is still in use.

Article 24 allows nations to adopt an "interim agreement" for up to 10 years while they are drawing up a customs union or free trade area.

But there needs to be a "plan and schedule" in place to get towards an end-state agreement.

(Image: AFP/Getty Images)

Despite this Boris Johnson claims Britain can get a "standstill" arrangement from November 1 onwards under GATT 24 to prevent crippling new tariffs.

He says the option is his 'Plan B' and offers a possible "way forward" with "agreement on both sides". Plan A is to renegotiate the Brexit deal and come out with a deal on October 31.

If Plans A and B both fail, then Plan C is to come out of the EU with no standstill arrangement and no deal at all on October 31.

It came as Boris Johnson and Jeremy Hunt clashed over the deadline for Brexit tonight - with Mr Hunt refusing to guarantee EU withdrawal by Christmas.

Foreign Secretary Mr Hunt has come under mounting pressure to commit to quitting the bloc by October 31, the latest deadline.

Rival Mr Johnson warned it would be “absolutely insane now to say that yet again we have a phoney deadline ... kick the can down the road”.

But pressed in a 30-minute BBC interview if Brexit would happen before Christmas, Mr Hunt would only say: “I expect so.”

(Image: AFP/Getty Images)

Jeremy Hunt later stood by his remarks refusing to guarantee Brexit by Christmas.

Speaking at a Tory hustings in Cheltenham, he said: “No Prime Minister, no person who wants to be Prime Minister, should make promises unless they know they can keep them.”

Meanwhile Andrew Neil skewered bookies’ favourite Mr Johnson on whether he would suspended Parliament, called “proroguing”, to force through a no-deal departure by Halloween .

“I don’t think it will be necessary to do anything like proroguing Parliament," he said - but once again didn't rule it out.

It came as police launched a probe into the leak of confidential memos branding the White House "inept" that led to Britain's Ambassador to the US standing down.

Grilled on the row, Boris Johnson finally admitted he should have stood up more for Sir Kim Darroch - after being heckled by a furious Tory member.

(Image: REUTERS)

The unknown member shouted “answer the bloody question!” at a hustings tonight as Mr Johnson dodged saying if he regretted not guaranteeing Sir Kim Darroch’s job.

The Tory leadership favourite replied: “Well, I don’t think that anything I said was actually decisive in Kim’s decision to resign.

“Had I my time again, to answer your question directly, yes - I probably should have been more emphatic that Kim personally had my full support.

“But I was surprised that his tenure as Ambassador in Washington should be raised by the Foreign Secretary as a fitting subject for debate in a Conservative Party leadership campaign.”