The article establishes that Brexit has put decentralization in the UK under serious threat. It is stated that there is a sharp gap between the way decentralization agreements are interpreted in Westminster and the way they are interpreted in Wales, Scotland and Northern Ireland. Brexit has caused serious divisions between the main ruling parties in Northern Ireland - Sinn Féin, which heads the regional government and advocates a reunited Ireland, and the Democratic Unionist Party (DUP), which advocates the region remaining within the UK. The article explores how the UK governments could conclude new UK-wide agreements. The focus is on the environment, agriculture and fisheries, three policy areas in which the powers devolved to the regions in the process of decentralization are in significant conflict with post-Brexit EU law. It is proved that decentralization in the United Kingdom is closely linked to its membership of the EU, which provides an external system of support for internal governance. Brexit destabilizes the internal settlement and raises a number of serious constitutional issues. The UK's membership of the EU has reduced the possibilities for secession and independence in both Scotland and Northern Ireland. This is because the seceding territories were supposed to share an economic and regulatory space with England. Thus, the vertical scaling up to the EU and down to the subnational level largely coincided. The article establishes that the decentralization settlement was achieved during the UK's membership of the EU and is deeply rooted in it at the legislative level. For those in favour of Brexit, EU membership contradicts the unitary vision of the state based on the supremacy of parliament. However, if we consider the UK as a union, there are many similarities between it and the EU. The development of relations between the government and civil society plays a significant role in the decentralization process, which can be useful for Ukraine in the context of its integration processes into the EU.