As we mentioned, GATT 24 is one of a number of obscure pieces of trade law to have taken on near mythical status in the Brexit debate.

If you don't quite feel fully up to speed, you can find out more here.

The candidates are asked to raise their hands if they are confident in telling people the UK will leave the EU on 31 October.

Both raise their hands.

"That's the spirit Jeremy," Mr Johnson quips. Mr Hunt shouts over the audience's reaction: "And I'm the man to deliver it!"

Reality Check

Image source, EPA

Jeremy Hunt has set out his solution for avoiding checks on the Irish border: Mobile checks for food products; a trusted trader scheme; and existing technology.

The EU has already cast doubt on the use of technology and other alternatives. Michel Barnier, the EU chief negotiator, said: "None of these arrangements are operational today. You need technical infrastructure to do that. And this takes time.”

You can read more on why Irish border has been a major sticking point in the Brexit negotiations here:

• Brexit and the Irish border: A simple explanation

Mr Johnson says he won't team up with The Brexit Party.

He already has the support of more than half Conservative Party MPs, he says, arguing: "We can deliver Brexit ourselves."

Mr Hunt repeats his message that the new PM should be the best negotiator.

Reality Check

Boris Johnson has said a number of times that a previously obscure piece of trade law could allow the UK to continue to trade with the EU without tariffs in the event of a no-deal Brexit.

Article 24 of the General Agreement on Tariffs and Trade (GATT) can be used to keep things the same for up to 10 years, while two sides negotiate a permanent future trade agreement.

But a trade agreement has to be agreed in principle before it can be used - that means both sides need to agree. In effect, for it to work, there would have to be some kind of deal.

This evening's host is being praised on Twitter. A few moments ago ITV presenter Julie Etchingham refused to allow the two rivals to talk over one another.

Here's how some of those watching responded...

Mr Johnson says it is "totally defeatist not to set a hard deadline" for Brexit.

But Mr Hunt tells his rival: "The difference between you and me is you are peddling optimism.

"We need leadership that is going to guide us through a constitutional crisis... and that means being honest with people" about the risks of no-deal Brexit.

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Hitting back at those criticisms of his no deal position, Mr Johnson says: "If we are going to have a deadline, we must stick to it.

"The more we prepare the less risk of disruption."

Mr Hunt says: "My point is you (Mr Johnson) said we would leave with no deal, no tariffs - that is wrong."

He says Mr Johnson's claim that the chances of a no deal are "a million to one" was "unrealistic" too.

"What you are saying is business doesn't need to prepare - they do need to prepare. The chances of no deal are much much higher than that," he adds.

The men are asked about no deal and the potential effect it could have on business.

"I don't think we are going to end up with a no-deal exit," Mr Johnson says, but the "crucial thing" is to prepare for one anyway. He acknowledges that no deal handled badly could be damaging though.

Mr Hunt says to the questioner: "I ran my own business just like you do...

"We can get through this... we can only get through this if we have a plan."

He says he has a 10-point Brexit plan.

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Boris Johnson says he will "solve the problems of the Irish border where they properly belong, in the context of the free trade agreement, that we will do after we come out on 31 October".

The EU has so far insisted it won't agree a withdrawal deal without an arrangement for the Irish border.

And it's said in the context of a no-deal Brexit, it will not begin discussing a future trade deal until the issue of the Irish border is settled.

The debate is heating up.

Mr Hunt says his worry is the UK is setting a "fake deadline" with 31 October.

He insists "You are more likely to get Brexit with me" because his rival's strategy is most likely to result in a general election first.

Mr Johnson says the EU will not take the UK seriously if no deal is off the table.

"It's not do or die is it? It's Boris in Number 10 that matters," Mr Hunt adds.

Jeremy Hunt asks whether Mr Johnson will resign if he doesn't deliver Brexit by 31 October.

Mr Johnson says delay does not deliver a deal, a deadline delivers a deal.

Mr Hunt says: "I think that's a no."

It's getting quite heated quite quickly...

"My opponent is clearly not committed to coming out of the EU on 31 October," Mr Johnson says.

"I think it is very, very important not to envisage any circumstances in which we would fail to come out of the EU."