The two leaders now argue over whether the union is at risk because of Brexit.

Boris Johnson says Labour would agree to another referendum on Scotland to get the support of the SNP and that isn't a price he would be willing to pay.

Mr Corbyn says he wants to reply to the "nonsense" from the PM on that issue, saying there is not going to be a coalition between his party and the SNP.

"Our country is obviously very, very important and we have to bring this business to a close," he says.

The SNP has said it would be willing to work with Labour by the way, but has suggested another independence referendum would have to be in play.

There are about 200 people in the audience. They are asking the questions. And there have been a few bursts of applause for both party leaders during these exchanges on Brexit.

Reality Check

Jeremy Corbyn has just said that Boris Johnson's deal "narrowly got through the House of Commons with the support of the DUP".

That's not right - Mr Johnson's deal was supported by 285 Conservative MPs, 19 Labour MPs and 25 independents.

All 10 DUP MPs voted against the deal.

Nick Eardley
BBC political correspondent

The first big clash between the two there on the NHS. It’s one of the issues that the public cares about most according to polls.

The Labour leader says his Brexit policy "is very clear" - three months to negotiate, six months for a referendum.

He hits back at the PM, accusing the Conservative government of holding "secret meetings" with US negotiators offering "full market access" the NHS.

Brandishing a document, he hammers home his message that the PM is putting the health service at risk.

But Mr Johnson says it is "an absolute invention" and says: "Our NHS will never be for sale."

The PM adds: "The only reason it comes up is to conceal the void at the heart of [Mr Corbyn's] Brexit policy and answer the question which side would he take."

Nick Eardley
BBC political correspondent

Jeremy Corbyn is trying to counter Mr Johnson’s message that he can get Brexit sorted by claiming there will be years of further negotiation to come.

Can he chip away at the idea of getting Brexit done?

Mr Johnson pushes again at the idea that the Labour leader isn't telling the people whether he wants to leave the EU or not. He asks "what is the point of Brussels offering a deal" if Mr Corbyn might campaign against it.

The Labour leader says: "I want to bring people together so therefore there will be a referendum."

He says Mr Johnson cannot get a trade deal with the EU done in the next few months as he has promised, adding: "You know that perfectly well."

But the PM says he has "ample time" to get his deal done.

Mr Corbyn points out that the EU's trade deal with Canada took seven years to do.

Reality Check

Just minutes into the debate, Boris Johnson has used the phrase "get Brexit done" or "getting Brexit done" several times.

So will Brexit be "done" by 31 January?

That depends on what you mean by "done". If the government can push through Mr Johnson's deal then the UK will leave the EU but it would be the beginning of an extremely complicated process.

The UK would still follow EU rules and pay into the budget until the end of the "transition period" on 31 December 2020. During this period, the UK will have to negotiate a free trade deal with the EU and there is no guarantee it would be done before the transition period ends.

At the same time, the UK will also need to negotiate other parts of its future relationship with the EU, including data and security.

Read our full Reality Check analysis here.

Nick Eardley
BBC political correspondent

This is the line of attack we expected from Mr Johnson. He believes a key weakness is that Mr Corbyn hasn’t said how he’ll campaign in another referendum.

Mr Johnson criticises the proposal by Labour to renegotiate a deal with the EU and attacks Mr Corbyn for not saying whether he would back Leave or Remain in a further referendum.

The Labour leader says he will "abide by that result", but does not say which way he would vote.

But Jeremy Corbyn rejects the idea he offers uncertainty. He says it will only take Labour three months to negotiate a better deal with the EU before putting it to another referendum.

He says the idea that the prime minster can get a trade deal done with the EU by the end of 2020 is "such nonsense".

Mr Corbyn says: "It is time we recognise we have to maintain a firm trading relationship with Europe."

The first half of the ITV programme is focusing on Brexit.

And the first question from the audience asks both leaders about the timetable for leaving the EU - "can you reassure us we won't be talking about this forever," the questioner says.

Mr Johnson insists we will "certainly come out on 31 January".

He says his deal with Brussels is approved by every Conservative candidate and "delivers everything we were promised".

He adds there is a sharp distinction between his deal and the "dither and delay" offered by Jeremy Corbyn.

Nick Eardley
BBC political correspondent

Both leaders were sticking to their main campaign messages in their opening statements and getting some key soundbites out.

Jeremy Corbyn says it’s real change and real hope on offer - and a Labour government for the many not the few.

Boris Johnson wants voters to blame a deadlocked parliament for a winter election - and argues it’s he who will “get Brexit done”.

Boris Johnson is up next with his 60-second opening statement.

The PM says the only reason the election is taking place is because of "deadlock in Parliament".

He says his Brexit deal is "ready to go", adding: "As soon as we can get that deal through Parliament, we can get on with the people's priorities."

In his opening statement, Jeremy Corbyn says this election "gives you a real choice".

The Labour leader says: "A choice about your future, the future of your community and of your country.

"Labour is offering real change and real hope."

Here we go.

Boris Johnson and Jeremy Corbyn have taken to the stage as ITV's Julie Etchingham introduces the start of the show.

The debate will last an hour and it is the Labour leader up first with his opening statement...

Nicholas Watt
Political editor, BBC Newsnight

In nearly 10 years of UK general election TV debates only one leader has really knocked it out of the park. Step forward Nick Clegg in 2010....who then lost five seats despite #cleggmania.

And Nick Clegg’s great strength in 2010 was his novelty. Unlike Clegg, Boris Johnson and Jeremy Corbyn are very well-known.

A last-minute video from Boris Johnson, who says he's looking forward to getting across the Conservative Party's "key messages" in the debate.

Laura Kuenssberg
BBC political editor

These events present huge risks and opportunities.

Both parties aware tonight is a chance for Labour to start narrowing the gap in the polls more convincingly, or for Tories to consolidate and start pulling away. They also both know their candidates delight and divide in equal measure.

Primetime TV is a very powerful forum to make an impact, good or bad, with no distractions and no other competitors, and a studio audience. There'll be no hiding, especially with the excellent Julie Etchingham in charge.