Former British Prime Minister John Major has argued that Brexit could lead to a hard border as the European Union and the UK must control their borders for customs purposes.  The Conservative Party`s European research group believes that the UK could have the choice of not controlling its border if VAT is not applied, or of controlling the border in order to apply a possible VAT to goods imported after Brexit.   A week after the Brexit referendum, then-Northern Ireland Premier Arlene Foster of the DUP and Deputy Prime Minister Martin McGuinness issued a joint letter stating that the border should not become a catalyst for illegal activity or create an incentive for those who want to undermine the peace process.  the extent to which BRITISH governments have benefited from the political autonomy that would theoretically be granted to them outside the EU. If British policy differs significantly from that of the EU in key areas, it could complicate cross-border cooperation.56 Political relations in Northern Ireland could be further undermined by the power-sharing government that forces the two largest parties, currently the DUP and Sinn Féin, to share the first ministerial post in a coalition. Given that the DUP campaigned for Brexit while Sinn Féin backed Remain, this could lead to problems even if the positions of the DUP and Sinn Féin in the debate on the EU referendum were predictable. 29As never a threat to the peace process, Brexit could also be a source of instability for the border between Northern Ireland and the Republic. Thanks to EU membership (including the introduction of the single market in 1992) and the implementation of the peace process, the border between the two states has become a soft border. Since the abolition of customs controls in the 1990s, when customs controls were abolished, there has been almost no physical manifestation of the border.65 This soft border is the symbol of successful peacebuilding and of practical importance as it ensures freedom of movement between the two states. This certainly explains why this topic has been considered crucial by researchers. For example, the report published by the Northern Ireland Committee insisted that „an agreement that maintains a soft land border … but do not impose travel restrictions within the UK, should be a priority in the event of Brexit.“66.
In fact, Brexit could theoretically reintroduce a hard border, as Northern Ireland – unlike the Republic of Ireland – would no longer be part of the EU. The border between Northern Ireland and the Republic would become an external border of the EU and should be managed in such a way as to prevent the immigration of EU nationals to the United Kingdom. Therefore, border controls should be introduced. Michel Barnier, the EU`s chief negotiator, said he would work with the UK and Ireland to find „solutions“ to threats to Ireland`s trade relations, the common travel area and the Good Friday agreement.  Irish Foreign Secretary Simon Coveney denies British media reports that Ireland expects the effective border to become the Irish Sea, saying that „British officials would have their hands to find an imaginative solution, but [the Irish government] would not support a proposal that would see a hard border return to Ireland.“  In Northern Ireland, the debate on the EU referendum focused on two specific issues related to the political stability of the province and the Irish border, in particular with regard to the Remainers. On the one hand, there were fears that leaving the EU would jeopardise the peace process, as it undermines the decentralisation agreement signed in April 1998 (Good Friday Agreement). .