This thesis is intended to answer the adverse effects of the implementation of Hard Brexit on the economy of the United Kingdom. Hard Brexit means that the membership withdrawal of the UK from the European Union, including its Single Market and Customs Union. Another term for Hard Brexit is Clean Brexit, which has the same meaning and has been used interchangeably by mass media in addressing the issue. The British Prime Minister, Theresa May, took this decision (implementing Hard Brexit) because she wanted to grant the wishes of the majority of British citizens. They wanted the UK to stop sending money to the EU budget and to be free from the EU’s grip, so the country can sign trade deals with other countries anywhere in the world without the bloc’s intervention. In other words, the UK’s economy will be more prosperous by implementing Hard Brexit when leaving the Union. But in reality, leaving the European Single Market and Customs Union will make the UK’s economy much worse, because its economy is already deeply integrated and interconnected with its EU counterparts as the result of more than forty years EU membership. In 2017, the UK exported 44.5% of goods to the EU and imported 53.1% from them, Britain needs the EU and its market, not the other way around. At this time, the UK has not officially left the EU yet, but the adverse effects of Hard Brexit already emerged. The loss of FDI on the British aerospace and automotive industries, JRL may close its UK car plant and hold any investment for the UK, BMW may close its Cowley plant and move its Mini production to the Netherlands, Nissan’s cancellation to manufacture its X-Trail model in its UK car plant, Honda’s plan to close its Swindon plant in 2021, and other negative effects. Therefore, in the thesis, I argue that implementing Hard Brexit will bring harm and unimaginable losses to the British economy. In writing this thesis, I use the theory of Regionalism and its sub-theory, Regional Trade Agreement (RTA) to answer 2 research questions. The EU is one of the regional organizations in the world that implements two types of RTA, Single Market and Customs Union. Therefore, I think Regionalism and its RTA are the right theory and sub-theory to answer the research questions written in the thesis.

Key words: Brexit, Hard Brexit, Single Market, Customs Union.