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During the transition period the UK and the EU plan to negotiate a comprehensive future relationship that will maintain a fully open Irish border.

The backstop is a kind of safety net - a way of avoiding a hard border if the future deal has not been agreed in time.

In the withdrawal agreement, it is set out as a single customs territory covering the EU and the whole of the UK, but Northern Ireland would also have to follow other EU rules.

The EU and UK have said that the backstop should only be a temporary arrangement until a permanent solution is found.

But critics point out that it is not time-limited in the proposal and the UK cannot leave it unilaterally.

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SNP Westminster leader Ian Blackford says it is an "absolute travesty" that MPs are only being given the choice between the government's deal or no deal, when an alternative could be considered.

"There is no such thing as a good Brexit," he says. "It is the responsibility of any government to provide security for its citizens."

The government is "putting workers on the dole as a function of ideology" through their Brexit approach, he says, with "an eternal battle in the Tory party" although that has happened since the 2017 general election.

The SNP are "particularly alarmed" by the end of freedom of movement, he says, "this is about people who are part of us, our friends, our relatives", whilst British people also will miss out as a result.

"Why should my grandchildren not have the same rights that my generation had?"

Scotland has an ageing population, he says: "The last thing we need is to be cut off from that supply of labour."

He urges the Labour party to "get off the fence", warning young people will never forgive them if they do not: "this is the opportunity to unite, to unite this house, to vote down the government's deal and to support a 'People's Vote'."

Scottish people were told in the 2014 independence referendum that their desire to stay in the UK and the EU would be respected, he says, but the same desire has not been respected after they voted to remain in the EU in 2016.

She's referring to the system in Parliament for MPs who cannot make a vote, called "pairing". It's where an MP on either side of an issue agree not to vote, so they cancel each other out.

However, Ms Siddiq said she did not trust the system after Tory chairman Brandon Lewis was accused of breaking the system when he voted on a Trade Bill, despite being paired with Lib Dem Jo Swinson, who was on maternity leave.

He later apologised for making an "honest mistake".

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Father of the House and Tory MP Ken Clarke is the first backbench MP called to speak in the last day of the Brexit debate.

He begins by saying the debate is "chaotic", and that younger generations will look back and wonder how this happened.

Mr Clarke adds that he is "a hardline Remainer" and will be supporting the withdrawal agreement as it is the "harmless" exit paper "before the real negotiations" start.

The House needs "some kind of consensus", he adds, telling MPs who are moaning about the backstop that they must be "suffering from some form of paranoia to think the Irish backstop is some plot to keep the British in the EU".

Concluding his speech, Mr Clarke says the government "must get on to the real negotiations" and it is "factually impossible" to get another withdrawal agreement ready by 29 March if Theresa May's deal is rejected today.

If the deal fails, he calls for Article 50 to be revoked "as a means of delay", and for it to be invoked again when there is a consensus in the House.

The Attorney General Geoffrey Cox says Article 50 cannot be revoked unless the UK cancels its departure from the EU.

Victoria Derbyshire

That's a question the Victoria Derbyshire programme has been asking. Here's a handy video summing up the Brexit story - so far.

BBC Politics Live
BBC2's lunchtime political programme

But the comment has attracted criticism from some Labour MPs:

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Labour's Nick Thomas-Symonds says the attorney general is "clutching at straws" with his arguments defending the backstop.

Potential future cases for breach of good faith or fundamental changes in agreements are both weak arguments that "do not wash with anyone".

"I wish there was a withdrawal agreement that was worthy of support across this House... the prime minister says she wants to unite, but all she has done is divide."

He says the prime minister has had two years to reach across Parliament and build a consensus: "She has failed to do so."

"The country deserves so much better than this totally inadequate agreement...we can and must do better."

Labour MP Lisa Nandy has also expressed her frustration:

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Michael Gove intervenes during shadow solicitor general Nick Thomas-Symonds speech to defend Theresa May's deal.

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Leader of the Labour Party Jeremy Corbyn, shadow home secretary Diane Abbott and shadow digital, culture, media and sport secretary Tom Watson support the shadow solicitor general Nick Thomas-Symonds as he makes his speech.

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Shadow solicitor general Nick Thomas-Symonds says that the Attorney General has previously told MPs to "grow up" and that there was "nothing to see here" in his legal advice to the government on the backstop.

He adds that the government was found in contempt of Parliament over the AG's advice, for failing to publish it when originally asked for.

Are MPs to assume that the government will be able to negotiate a full trade deal before the backstop has to come into effect? he asks.

He says Labour cares "deeply" about the "livelihoods, communities" which live along the Northern Irish border. He says that Labour has a commitment to the Good Friday Agreement.

He argues that the bespoke customs union in the deal has "no formal governance" and the problems caused by it "would see the United Kingdom fall behind over time".

If you're planning to follow this evening's vote as it happens, here's a bit more on the timings.

Of course we'll be covering it live here on the BBC news website as well.

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It makes no sense to reject the withdrawal agreement on the grounds that the agreement provides insufficient certainty when no alternative is on offer, Attorney General Geoffrey Cox says.

The political declaration provides "clear parameters" for the future relationship, he adds:

• No free movement of people
• No membership of the single market
• Independent trade policy for the UK

Labour MP Rachel Reeves criticises he has focused his remarks on the backstop, which she says is the main concern of Tory MPs but not the majority. She urges the government to rule out a no-deal Brexit.

Mr Cox responds that if MPs want to avoid no deal, they should support the agreement.

"We must seize this opportunity now, this is the key by which we unlock our future outside the European Union.

"When historians come to write of this movement, future ages will marvel at the huge reparation of powers," he says.

Image source, Getty

What is the Withdrawal Agreement?

Theresa May has agreed a deal with the EU on the terms of the UK's departure. It includes how much money the UK must pay to the EU as a settlement, details of the transition period, and citizens' rights.

It also covers the so-called 'backstop', which ensures that no hard border exists between Northern Ireland and the Republic of Ireland after Brexit.