No sooner had he taken office than the UK’s new Prime Minister Boris Johnson demanded that the European Union ditch its insistence on the Irish border backstop. The measure has long bedevilled Brexit negotiations and was one of the main reasons why the negotiated divorce agreement was rejected by the British parliament. Hardline Brexiters fear it could effectively keep the UK tied to the EU indefinitely. The backstop is essentially an insurance policy to avoid a hard border — such as border posts — between Northern Ireland (part of the UK) and the Republic of Ireland (part of the EU), post-Brexit. The UK is due to leave the EU on October 31. Failure to agree terms means a “no-deal” exit is likely which would be damaging to both sides. Johnson’s demand was immediately rebuffed by the EU’s chief negotiator Michel Barnier, who called it “unacceptable”. The UK voted to leave the EU by 52% to 48% in a June 2016 referendum, but Northern Ireland voted by 56% to remain in the EU. After Brexit, the frontier between Northern Ireland and the Republic of Ireland will become the only land border between the UK and the EU. It is one of the most politically sensitive borders in the world. Decades of political and sectarian violence saw the UK install heavy security infrastructure including watchtowers, and army and police posts — which were often targeted by Irish Republicans. The period known as Northern Ireland’s “Troubles” culminated in a peace accord signed in 1998. Among the measures in the Good Friday Agreement, the UK government agreed to demilitarise and remove security installations. The UK and Ireland committed to “close cooperation… as partners in the European Union”. Both countries’ EU membership enabled the border to remain open. READ: Why is the Irish border issue so complex? | Euronews answers During the Brexit negotiations both the UK and the EU agreed that in all circumstances, no new physical checks or infrastructure should be introduced. However, the UK made it clear it wanted to leave the EU’s single market and customs union — and pursue an independent trade policy, potentially diverging on matters such as tariffs and regulatory standards. If both sides apply different regulations and customs rules, then enforcing those rules might mean checks and controls. Although some could be carried out away from the border, the 500-kilometre frontier between Northern Ireland and the Republic presents a formidable challenge. The backstop was conceived as a guarantee that the Irish border would remain open after Brexit. Although the UK wants a free trade agreement with the EU, it would apply until an alternative solution could be found. After 18 months of fraught negotiations between London and Brussels, the backstop mechanism was included in the legally-binding Withdrawal Agreement, approved by the British government and the other 27 EU countries in November 2018. Among other matters, the agreement paves the way for a transition period to come into effect after the UK’s departure — keeping many existing arrangements in place to avoid instant disruption and allow more time to sort out future UK-EU relations. Should future talks fail to produce a free trade agreement by the end of the transition period, at that point the backstop would come into force. It envisages the UK remaining in a “single customs territory” with the EU, with Northern Ireland more deeply integrated with the bloc’s rules. This should ensure the free circulation of goods — while the alignment of rules and regulations should avoid the need for physical border infrastructure, with checks and controls. For full details, see the section on the backstop here: READ: What’s in Theresa May’s Brexit deal and why is it so unpopular? During the negotiations leading to the Withdrawal Agreement, Irish and EU officials first recommended that to ensure an open border, Northern Ireland would effectively have to remain in the single market for goods and the customs union. A draft legal text envisaged that Northern Ireland and the Republic would effectively follow the same rules on trade. ‘Splitting off’ Northern Ireland from the UK Northern Ireland’s Democratic Unionist Party (DUP), on whose support the UK government has depended since 2017, has vehemently opposed any move it believes might separate Northern Ireland from the rest of the United Kingdom. Theresa May, then prime minister, complained that this would “threaten the constitutional integrity of the UK by creating a customs and regulatory border down the Irish Sea, and no UK prime minister could ever agree to it”. Keeping the UK ‘trapped’ in the EU The final text included in the Withdrawal Agreement sets out a UK-wide customs union with the EU. Earlier, London had proposed a “temporary customs arrangement” between the EU and the UK as a whole — not just Northern Ireland — to try to break the impasse. The arrangements enraged Eurosceptics in the DUP and Britain’s ruling Conservative Party. They suspect the backstop would keep the UK strapped permanently to EU trade policy, and also hated the fact that the UK would not be able to leave the backstop unilaterally. EU officials have pointed out that the UK-wide mechanism in the withdrawal agreement was included at the UK’s request, to avoid splitting off Northern Ireland from the rest of the UK. Although the UK parliament rejected the Withdrawal Agreement three times, MPs did back a move to seek “alternative arrangements” to the Irish backstop. Although they agreed to consider other alternatives in the future, EU leaders insisted the backstop must remain in the exit deal. Boris Johnson argues that technology can help maintain an open border, though many trade experts say nowhere in the world have such solutions yet brought a completely open frontier between two different regulatory and customs regimes. READ: What ‘alternative arrangements’ are there to the Irish backstop? There has long been a feeling in Ireland that many people in Britain underestimate the backstop’s significance. Commentators have argued that it is vital to preserve peace in a region blighted by violence in the past. For the EU, the backstop’s importance grew during the Brexit negotiations. Officials came to realise that the UK’s departure raised questions not just over trade, but also cross-border arrangements in many other areas. It was feared that cooperation might be disrupted to such an extent as to threaten the terms of the Good Friday Agreement. The EU also says it needs the back stop to protect the integrity of its single market and customs union. Boris Johnson says he is committed to “getting rid” of the “divisive” and “anti-democratic” backstop, arguing it threatens the UK’s independence and self-respect. London argued throughout the negotiations — a stance also taken by the new Johnson government — that the Irish border should be dealt with in future trade talks. Dublin and EU negotiators continue to insist that the open frontier guarantee, as set out in the backstop arrangement, must be included in the divorce settlement. Michel Barnier wrote to EU member states to say that “eliminating the backstop” from the agreement, as demanded by Johnson, is “unacceptable and not within the mandate of the European Council”. Watch Ken Murray’s report on the Irish backstop for Euronews in the video player above. READ: No-deal Brexit: everything you need to know READ: Brexit Guide: where are we now?