What is the backstop?

The backstop is, in short, an insurance policy.

It spells out what should happen if the UK and EU cannot agree a comprehensive trade deal that would maintain an open border between Northern Ireland and the Republic of Ireland.

Both sides say they do not want the backstop to ever become a reality, because they hope to reach a deal that would avoid a hard border.

However, the EU insisted on a fall-back to ensure the border will remain open even if no deal on the future relationship between the EU and UK is agreed.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

The Irish government is particularly concerned that an open border be maintained, given it is a key part of the 1998 Good Friday Agreement that brought peace to the island of Ireland.

What would the backstop involve?

The backstop would see the UK enter into a temporary customs union with the EU. This would ensure there is no need for customs checks on goods travelling between Northern Ireland and the Republic of Ireland.

Northern Ireland would also abide by EU single market rules on goods. That would mean there would also be no need for regulatory checks of products crossing the border, allowing an open border to be maintained.

However – and there is always a however – the fact that Northern Ireland would effectively remain in the single market, while the rest of the UK would not, means there would have to to be new checks on goods travelling between the region and the rest of the UK. The EU has said most of these checks could be carried out in factories and farms rather than at the border, but for goods such as live animals this would not be possible.

The EU had initially pushed for the backstop to apply only to Northern Ireland, but Theresa May refused, saying she would not accept any situation in which the region had different customs rules to the rest of the UK.

Why is it so controversial?

The backstop is, by some way, the most contentious part of the Brexit deal. It was the subject of months of drawn out negotiations with the EU and is the main reason why so many MPs are opposing the proposed agreement, forcing Ms May to delay the House of Commons vote on her deal.

According to its critics, there are three main problems with the backstop.

Firstly, the UK would not be able to exit the mechanism without the EU’s approval.

After Brussels rebuffed Ms May’s attempts to secure a unilateral exit clause, the draft withdrawal agreement says the EU and the UK must “decide jointly” if and when the backstop is no longer necessary. If they disagree, the matter would go to arbitration.

Critics say this means the EU could keep the UK trapped in the backstop for as long as it wishes, with nothing the UK government could do about it.

The second point of contention is that the backstop is “indefinite”. There is no time limit on the protocol; instead, the withdrawal agreement says only that the provisions apply “unless and until they are superseded, in whole or in part, by a subsequent agreement”.

That is a major issue for Tory Brexiteers, who fear that the supposedly temporary arrangement could turn into the UK remaining in a customs union with the EU permanently, thus making it difficult to strike new trade deals with other countries.

The third problem is the one that most bothers the Democratic Unionist Party (DUP), which props up Ms May’s government in the Commons.

The Northern Irish party is furious that the backstop would see new regulatory checks introduced on some goods travelling between Northern Ireland and the rest of the UK. Staunch unionists fear this is the first step towards the break-up of the United Kingdom.

What does the government say?

The government insists there cannot be a withdrawal agreement of any kind without a backstop, because the EU demands one.

Ministers claim the protocol will not be needed and that neither side wants to see it implemented.

They say it will be temporary but admit that the UK would not be able to leave unilaterally.

The government’s legal advice, provided by attorney general Geoffrey Cox, said a permanent backstop would likely be illegal under EU law, and so could challenged – a point ministers have flagged in an (unsuccessful) attempt to win over Tory rebels.

Attorney General Geoffrey Cox: 'There is no unilateral right for either party to terminate Irish backstop if activated after the Brexit transition period'

Ms May has also said the UK would be able to choose whether to enter the backstop. This is because, if no trade deal is agreed by the summer of 2020, the British government will be able to express a preference as to whether to enter the backstop or extend the transition period to allow for further negotiations.

The transition is due to expire at the end of 2020, but the withdrawal agreement allows for it to be extended by up to two years. This would provide more time for a deal on the future relationship to be agreed, meaning the backstop would not be necessary.

However, the EU would need to approve the UK’s request. If the two sides disagree, the matter would go to arbitration. The UK could not, therefore, simply decide not to enter the backstop.

And if no deal is agreed at the end of the extension, the backstop would still be implemented. That is because it is the default unless and until there is something to replace it.

Ahead of the second meaningful vote, Ms May made an eleventh-hour dash to Strasbourg to secure three new documents that she says will give MPs the legally-binding reassurances they require to back her deal.

These include a “legally binding joint instrument”, which the government says will reduce the risk of the UK being held in the Northern Ireland backstop indefinitely, and commits both sides to replace the backstop “with alternative arrangements by December 2020”.

The package also includes a “unilateral declaration by the UK”, giving the government the right to take the EU to independent arbitration if it does not sincerely try to remove the backstop – and a political declaration setting out both sides’ commitments to speeding up the process.