As a growing economic power Japan experienced trade conflicts with the European Communities (EC; from 1993 onwards the European Union, EU). Although the British government was criticised by EU partners and domestics groups – politicians, industry, trade unions and public opinion – the UK became Japan’s primary gateway to the European single market. In 1973 Japanese multinationals began constructing manufacturing plants in Britain to reduce exports from Japan, to mitigate export duties and to ease these trade conflicts. Brexit, therefore, poses a serious risk and requires Japan to urgently reconsider and redefine its relations with Britain, the EU and other Member States of the EU. On the other hand, uneasy stand-offs in the Asia-Pacific region might push post-Brexit Britain to cooperate more closely with Japan in security matters. China’s territorial disputes with neighbouring countries and North Korea’s nuclear threat will require Japan and Britain to play a more active role in stabilising the region, not only by conducting joint military drills, but also by increasing collaboration in the arms industry. UK–Japan relations are drifting away from their traditional focus on trade and are expanding into the military field.