Meanwhile, a UK diplomat in charge of Brexit at the British embassy in the US has quit.

In her resignation letter, seen by broadcaster CNN, external, Alexandra Hall Hall said she could no longer "peddle half-truths" on behalf of political leaders she did not "trust".

She said she has become "dismayed" by the reluctance of politicians to "honestly" address the "challenges and trade-offs" involved in leaving the EU.

The Foreign Office said it would not comment on details of her resignation.

Read the full story here.

Image source, AFP

A bit more from former Labour PM Tony Blair at the Final Say rally.

He says an older generation have "no right" to steal the future from a younger generation who know that the world today depends on the connections which Brexit breaks.

Mr Blair says: "This election may determine a government lasting for five years. But its consequences may last for generations. Do not give Boris Johnson a majority. He does not deserve it."

He says some people groan at the idea of another hung Parliament but says it was better than a country "hung for want of leadership".

Mr Blair says he will be voting for Labour but urges people who support another referendum to look "constituency by constituency".

"It's not Brexit that's getting done. We're getting done. So here we are: crazy times. Strange bedfellows. Unique risks".

He says there has never been such a momentous choice for the British public. "I beg you, choose wisely," he says.

Jennifer Scott
BBC News

It's an odd feeling in the studio. A quiet calm before what could be a stormy debate between the two leaders.

Camera people are practising their moves while political bigwigs get a tour of what's on offer.

What's strange is how small it is - more school drama studio than Hollywood set.

But we're hoping for some exciting story lines tonight, and both Johnson and Corbyn will be hoping to come across as pros rather than amateurs.

Here are a few snaps from behind the scenes...

Tony Blair gets a laugh when he opens his remarks at the Vote for a Final Say Rally by saying: "Here are five words I never thought I would say: Thank God for John Major."

He says to Sir John and to Lord Heseltine: "For years I stood against you, tonight, it is an honour to stand with you."

He mentions David Gauke and other "good, smart, capable people across the political spectrum, including in the Labour Party" who are prepared "to put country before party and principle before position", adding that their "courage is an example".

He says people in the UK have "six days to save our future".

"Brexit is not a mistake. I wish it were. Brexit is a disaster," he says.

Promises on trade, the NHS, the economy and Northern Ireland demonstrate the "dangers of ever trusting the word of Boris Johnson".

Sir John says he believes leaving the EU is the worst foreign policy decision of his lifetime.

"It will affect every aspect of our lives for many decades to come and it will make our country poorer and weaker," he says. "It will hurt the most the people who have the least."

Sir John says never before have the stakes been higher in an election.

He urges people to make their vote count.

"Whatever you do, don't let your vote be wasted. Never has it been as important as it is now."

Sir John then introduces his successor as prime minister, Tony Blair.

Image source, Getty Images

More from that Final Say rally...

Former prime minister Sir John Major has also been addressing the crowd, via video link.

He says the future of the country is at stake in the upcoming election.

"When the nation voted on Brexit, it did so on the diet of fiction and undeliverable promises," he says.

Jennifer Scott
BBC News

Tonight's event is in a traditionally safe Tory seat - but that doesn't mean there isn't division between its voters.

Where better to get the knowledge then than the local cabbies?

One tells me he would be voting for Boris Johnson - not because he likes the PM, but because his rival is a "traitor to our country".

"I'll go with Johnson because you know where you stand with him.

"Look at who Corbyn's friends are," he says. "I don't want him in Downing Street inviting them round."

But one of his colleagues on the streets of Kent couldn't disagree more.

"Boris is just a liar, he can't even face Andrew Neil because he knows he can't cover it up," says another driver.

"I'd like to go with Corbyn, but I have to vote Lib Dem here. They are the only ones who aren't lying about their plans and it is the only way to get Boris out."

The Vote for a Final Say rally - calling for a new EU referendum - has begun and former justice secretary David Gauke is up first.

BBC News Channel

On the BBC News Channel, presenter Christian Fraser takes a quick tour of the set for tonight's debate.

His verdict? It's "pretty slick", with podiums for the leaders closely placed in "quite an adversarial way" that could lead to "fireworks".

He points to the seating, saying the audience of just over 100 were chosen by pollsters Savanta ComRes to have equal representation from the Conservatives and Labour, with some undecided voters.

Nick Robinson, who will be moderating the debate, says the audience will ask the questions, but unlike Question Time, they won't follow them up.

"My job is to throw the questions to the two leaders try to get them to debate, but if they ignore the question or points that the other person makes, it’s my job to jump in and try to pin them down a little bit," he says.

"I’m not here to do an interview, I’m not the great interrogator, but I’m a little more than a referee - I'm trying to make sure it's free and fair and everyone gets an equal say."

He admits to "waking up in the middle of the night" thinking about the debate, saying it is a rare opportunity to hold the prime minister and leader of the opposition to account in this way.

"I don’t want to get in the way of them saying what they want to say, but I don’t want people at home saying, why did you let them get away with that?"

As we know, Boris Johnson will tonight go head-to-head with Labour leader Jeremy Corbyn in the final televised debate of the election campaign.

Tonight's hour-long Prime Ministerial Debate will be broadcast on BBC One at 20:30.

But as we build up to our debate, we look back at how many people tuned into the first televised head-to-head between the two leaders.

The first head-to-head of the election campaign was Johnson v Corbyn: The ITV debate, which aired on 19 November.

Moderated by ITV news presenter Julie Etchingham, the hour-long programme garnered 7.3 million viewers, according to consolidated figures from the Broadcasters' Audience Research Board (BARB).

Perhaps surprisingly, it outstripped the likes of ITV soap Coronation Street and the BBC's His Dark Materials, which had seven million and 6.8 million viewers respectively.

A rally calling for a new EU referendum is due to begin shortly in London.

Former prime ministers Tony Blair and Sir John Major are among those who will be addressing the Vote For a Final Say event.

Aisling Ennis, from the Press Association, posted this image from the rally.

Image source, Getty Images / BBC

It's t-minus three hours until Boris Johnson and Jeremy Corbyn go head-to-head in a live BBC One debate - so grab your popcorn and get prepped. Here are six things you need to know:

1. The Conservative and Labour leaders will face each other for the final time during the election campaign in an hour-long debate hosted by Today presenter Nick Robinson

They will face questions from about 100 members of the public and from those who have submitted them via the BBC News website. It will be televised live in the UK on BBC One and on BBC iPlayer, and streamed live on the BBC News website, where you can also follow the latest reaction and analysis on our live page. It will also be broadcast live on BBC Radio 5 Live and on the BBC Sounds app.

2. Opinion polls suggest the Conservatives are still comfortably ahead of Labour

Mr Johnson "has to try his utmost not to slip up", BBC political correspondent Iain Watson points out, while Mr Corbyn may well be tempted "to focus as much on the personality of his opponent as on his own policies".

3. Mr Johnson is the first PM to agree to a head-to-head debate with his opponent

Margaret Thatcher didn't do it. Nor did Tony Blair. Neither did John Major or Theresa May. Prime ministers are always told not to risk giving their opponents the equal billing and status they crave, Robinson writes ahead of hosting the debate.

4. But he has yet to agree to be interviewed by the BBC's Andrew Neil

In a monologue on Thursday after an interview with Brexit Party leader Nigel Farage, Mr Neil issued a challenge to Mr Johnson and listed a series of questions that he wanted to put to him.

5. The two leaders are likely to clash over Brexit - again

The Conservatives have pledged to "get Brexit done" and leave the EU by 31 January. The Labour Party is promising to "get Brexit sorted" in six months by renegotiating Mr Johnson's deal with the EU and then putting that to a public vote.

6. The NHS, security, racism and climate change are also likely to feature heavily

The parties have clashed over whether it will be part of any post-Brexit trade negotiations with the US and UK security following the London Bridge attack. Both have had internal rows over claims of racism and made "green pledges" throughout the campaign.

Feeling a little confused about the election? Not sure why we're having another one when they're only supposed to happen every five years?

Let us walk you through it with our really simple guide.

Image source, Getty Images

There are just six days to go until you need to cast your vote, but how much do you know about the main parties fielding candidates in this election?

Here, in one place, is all the information you need about the seven main parties - and their leaders.

Will snow fall on election day on Thursday? It's too soon to know for certain, but if it does it would be the first time in 40 years that it has snowed on the day of a general election.

On that occasion, voters as far afield as Devon and Lancashire trekked to their polling stations in decidedly wintry conditions - despite it being 3 May.

Britain was in the middle of an Arctic blast, with snow, hail and ice affecting many areas.

Voters found themselves queuing in the snow in constituencies across south-west England, including the seat of Plymouth Devonport which was being defended by the foreign secretary David Owen.

You can see the forecast for your area next Thursday with the BBC Weather service.

If you're just joining us now, here's a quick round up of today's campaigning:

• Former Tory prime minister Sir John Major made a major intervention, urging voters to support MPs expelled from the Parliamentary Conservative Party for rebelling over Brexit. He is backing independent candidates David Gauke, Dominic Grieve and Anne Milton, all running against his party in the general election.
• Jeremy Corbyn released "leaked" documents, which he claims show Boris Johnson is "misrepresenting" the effects his Brexit deal will have on Northern Ireland. Mr Corbyn called the paper "hard evidence" NI would be "symbolically separated" from the rest of the UK, with customs checks on goods. Mr Johnson has repeatedly said there will be no border in the Irish Sea and dismissed Labour's claims as "complete nonsense".
• Both men are preparing for their head-to-head debate tonight, which is being hosted by the BBC's Nick Robinson. It will be live on BBC One at 20:30.
• The Tory candidate for Hastings is facing criticism for suggesting disabled people could be paid less than minimum wage, as some people with learning difficulties "don't understand about money".
• The PM said Mr Corbyn should reconsider plans to extend the franchise to EU nationals living in the UK if the Labour hold a new EU referendum.

Some reaction to the first glimpses of the BBC debate set for tonight is emerging online.

Mail Online political editor James Tapsfield says that while ITV's set earlier in the campaign recalled classic sci-fi movie Tron, the BBC's set looks a bit more like 1980s and 1990s ITV quiz show Blockbusters.

BBC Politics Live
BBC2's lunchtime political programme

On BBC Politics Live earlier today, business minister Nadhim Zahawi spoke about Labour's leak of a government document detailing Brexit consequences for Northern Ireland.

Jeremy Corbyn says the paper is "hard evidence" NI would be "symbolically separated" from the rest of the UK after Brexit, with customs checks on goods.

The Conservatives say the document is an "immediate assessment, not a detailed analysis" and it had not been written for "decision-making purposes" or been seen by the PM, the chancellor or "any of the senior officials involved in the negotiations".

"It is a desperate, desperate attempt by Jeremy Corbyn," says Zahawi. "It was a flash analysis after the deal was done by Boris Johnson."

BAKING NEWS: Prime Minister Boris Johnson sported a Brexit-themed apron as he handed out pastries from behind the counter at Grodzinski Bakery in Golders Green, North London.

Image source, PA Wire

He was also snapped clutching his ministerial red briefcase as he boarded a train at the capital's St Pancras station. He had been expected in Kent, where he was supposed to make a speech to supporters outside a pub - but the Conservatives cancelled the visit, saying it was for "logistical reasons".

Image source, PA Wire

That may well have disappointed the small group of protesters who had gathered there ahead of the planned visit.

Image source, Reuters

Labour leader Jeremy Corbyn was not far away from the prime minister today, delivering a speech in the capital. Here he holds up a leaked Treasury document that he said shows Mr Johnson is "misrepresenting" his Brexit deal and the "devastating" impact it will have on Northern Ireland.

Image source, EPA

Meanwhile, Liberal Democrat leader Jo Swinson took her campaign to a playgroup at Hiltingbury Community Centre in Hampshire, where she was promoting her party's pledge to give working families subsidised care for children from the age of nine months.

Image source, PA Wire

She even decorated this Christmas biscuit. She didn't quite stay within the lines with the green icing - but sometimes that's just the way the cookie crumbles.

Image source, PA Wire

Also today, Ann Widdecombe, a Brexit Party MEP and parliamentary candidate, spoke at a news conference in London.

Image source, PA Wire

Image source, Getty Images

TV election debates are still a bit of a novelty in the UK.

Each time an election is called, there's the question of whether any will take place, how they will be organised and who will take part.

Broadcasters are still experimenting - this time putting on two-way debates, seven-way debates, a climate change debate and a special youth audience debate, among other permutations.

So is there room for improvement? And, if so, what might that look and sound like?

Head here to read about six suggestions from experts, which include turning off the microphones, banning clapping and having robot fact-checkers.